{
  "id": "pneumonia",
  "title": "Pneumonia",
  "lang": "en",
  "source_lang": "en",
  "summary": "Pneumonia is an infection that inflames the air sacs of one or both lungs, usually caused by bacteria or viruses, and remains a leading cause of death worldwide.",
  "category": "health",
  "subcategory": "diseases",
  "type": "concept",
  "aliases": [
    "Lung infection"
  ],
  "generated_at": "2026-10-10",
  "model": "pro/gpt-6.1-sol",
  "url": "https://aiwiki.page/en/pneumonia/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/pneumonia/",
      "json": "https://aiwiki.page/zh/pneumonia.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "Pneumonia is an infection of the lung in which the alveoli, the tiny air sacs where oxygen passes into the blood, fill with inflammatory cells and fluid, impairing gas exchange. It is most often caused by bacteria or viruses, with fungi and less common agents such as parasites, aspirated material or noninfectious irritants accounting for the rest. Illness ranges from mild cases managed at home to life-threatening respiratory failure and sepsis, and clinicians classify it by where it was acquired, by its probable cause, and by the patient's severity and risk profile.\n\nPneumonia matters because it is among the most important infectious causes of death worldwide, with the heaviest burden falling on young children, older adults and people in low-income settings. Diagnosis rests on clinical assessment supported by chest imaging and laboratory tests, while treatment depends on the cause and severity: antibiotics for bacterial disease, specific antiviral drugs for some viral infections, and oxygen and intensive care for severe cases. Prevention relies on vaccination, hygiene, reduced exposure to smoke and air pollution, and better management of chronic illness.\n\nThis entry covers the pathophysiology of the disease, its classification by setting and cause, host risk factors, clinical features and complications, diagnosis and severity assessment, treatment, age-specific considerations, prevention, global epidemiology, antimicrobial resistance, historical development, and the questions that remain unresolved."
    },
    {
      "heading": "Definition, scope and terminology",
      "text": "Pneumonia is inflammation of the lung’s gas-exchanging tissue, particularly the alveoli, or microscopic air sacs, and the surrounding tissue. In ordinary clinical usage, the term usually denotes an infection caused by bacteria, viruses or fungi. The inflammatory response can cause normally air-filled alveoli to accumulate fluid and pus, interfering with oxygen exchange. Pneumonia is distinguished anatomically from bronchitis, which primarily affects the larger conducting airways, and bronchiolitis, which primarily affects the small airways. These conditions belong to the broader group of lower respiratory tract infections, but they are not interchangeable diagnoses; the same respiratory virus can cause different illnesses depending on which structures are affected. (nhlbi.nih.gov)\n\nThe operational definition of a pneumonia case depends on the purpose and setting of assessment. The 2019 American Thoracic Society–Infectious Diseases Society of America guideline, led by Joshua Metlay and Grant Waterer, addressed adults with compatible signs and symptoms plus radiographic confirmation. Such a definition identifies a different population from one based on symptoms and examination alone. By contrast, the World Health Organization uses a clinical classification for children under five: cough or difficult breathing, with or without fever, accompanied by fast breathing or lower chest-wall indrawing can qualify as pneumonia without an X-ray. These definitions support different clinical tasks and should not be treated as equivalent measures when comparing studies or health-service records. (pmc.ncbi.nlm.nih.gov)\n\nIn epidemiology, “lower respiratory infection” may be an explicitly defined statistical category rather than a synonym for radiographically confirmed pneumonia. For example, the Global Burden of Disease 2021 methods appendix used clinician-diagnosed pneumonia or bronchiolitis as its reference case definition for nonfatal lower respiratory infections. Hospital and insurance-claims records were adjusted to make them comparable with that reference definition, including adjustments for readmissions and multiple diagnoses. Consequently, a burden estimate labelled “lower respiratory infections” cannot automatically be interpreted as a count of pneumonia alone: its scope depends on the included diagnoses and the methods used to harmonize source data. (healthdata.org)\n\nPneumonitis denotes lung inflammation and is commonly used for noninfectious inflammatory disease or injury. A useful distinction is between aspiration pneumonia, an infection following entry of bacteria-containing material into the lungs, and aspiration pneumonitis, chemical injury following inhalation of gastric contents. These processes may overlap, and imaging alone does not distinguish them. Another example is hypersensitivity pneumonitis, in which inhaled substances such as mold or bird-derived material provoke an immune response rather than an ordinary pulmonary infection. Terminology is not completely uniform: established names such as “lipoid pneumonia” describe inflammation associated with inhaled or aspirated oils, illustrating that the word pneumonia does not invariably establish an infectious cause. (msdmanuals.com)\n\nSeveral familiar labels describe different dimensions of illness. “Walking pneumonia” is an informal description of relatively mild pneumonia in someone who remains active rather than staying in bed; it is commonly associated with Mycoplasma pneumoniae. “Atypical pneumonia” refers to pneumonia associated with organisms or features differing from the conventional bacterial pattern, including M. pneumoniae and Legionella pneumophila. It is not synonymous with mild illness: Legionnaires’ disease can be severe. “Double pneumonia” means bilateral pneumonia—pneumonia affecting both lungs—not two separate infections. Thus, “walking” describes the apparent clinical impact, “atypical” describes an etiological or clinical grouping, and “double” describes anatomical distribution. (cdc.gov)"
    },
    {
      "heading": "Pathophysiology in the lung",
      "text": "Infectious material reaches the lower airways principally through aspiration or inhalation, and less often through the bloodstream. Small-volume aspiration—microaspiration—transfers secretions containing nasopharyngeal and oropharyngeal bacteria into the airways and can occur during sleep in healthy people. Infection develops when the microbial burden or organisms’ virulence exceeds local clearance capacity. Inhaled droplets and aerosols provide another route, particularly for respiratory viruses; particles sufficiently small to escape deposition in the upper airways can reach the alveoli. Bloodborne spread can seed lung tissue from a distant infection, as in tricuspid-valve endocarditis, in which infected material enters the pulmonary circulation. These routes introduce organisms into the lung but do not invariably produce disease. (pmc.ncbi.nlm.nih.gov)\n\nMechanical defenses include mucus that traps particles, cilia that transport mucus toward the pharynx, and the cough reflex. Organisms reaching the alveoli encounter soluble antimicrobial factors and resident alveolar macrophages, which ingest and destroy microbes. In healthy lungs, macrophages account for approximately 95% of airspace leukocytes, whereas neutrophils constitute about 1%. These macrophages and airway epithelial cells recognize microbial components through pattern-recognition receptors, including Toll-like receptors. Antibodies, complement proteins, and surfactant-associated proteins can coat microbes and facilitate their uptake. This combination of physical clearance and innate immunity normally controls small inocula without extensive inflammation. (pmc.ncbi.nlm.nih.gov)\n\nWhen these defenses are overwhelmed, macrophages and epithelial cells release cytokines and chemokines that amplify inflammation. Interleukin-1 and tumor necrosis factor contribute to systemic responses such as fever, while CXCL8, also called interleukin-8, helps recruit neutrophils from pulmonary capillaries into infected airspaces. Neutrophils engulf organisms and kill them using reactive oxygen species and proteolytic enzymes. The same substances can damage surrounding tissue when released extracellularly. Microbial toxins and infection-induced epithelial cell death add to this injury, weakening the barrier between alveolar airspaces and the circulation. Consequently, the host response both limits infection and contributes to pulmonary dysfunction; more inflammation does not necessarily mean more effective microbial clearance. (pmc.ncbi.nlm.nih.gov)\n\nIncreased alveolar-capillary permeability allows protein-rich fluid to enter the airspaces. This exudate, together with neutrophils, cellular debris, and sometimes erythrocytes, replaces air and produces consolidation: affected lung becomes denser and appears as an opacity on chest radiographs. Inflammation may involve a large contiguous region or scattered areas around distal airways; infection can also injure the alveolar walls rather than producing uniform airspace filling. Damage to alveolar epithelial cells impairs surfactant function, promoting alveolar collapse, while inflammatory debris can obstruct small airways. Fluid accumulation, collapse, and obstruction reduce the amount of lung available for ventilation and increase the work of breathing. (pmc.ncbi.nlm.nih.gov)\n\nGas exchange deteriorates because ventilation and blood flow become unevenly matched. Partly filled alveoli receive less fresh air despite continuing perfusion, producing a low ventilation–perfusion ratio, \\(V/Q\\). Completely unventilated but perfused units create an intrapulmonary shunt: blood passes through the lung without adequate oxygenation and then mixes with oxygenated blood. This is a major mechanism of hypoxemia, or reduced arterial oxygen, rather than simply slower oxygen diffusion through inflamed tissue. Carbon dioxide retention is not inevitable: increased ventilation can initially keep arterial carbon dioxide normal or low. In severe disease, however, increased respiratory loading and inadequate respiratory-muscle capacity can reduce effective alveolar ventilation, causing carbon dioxide accumulation and respiratory acidosis. (pubmed.ncbi.nlm.nih.gov)"
    },
    {
      "heading": "Classification by acquisition setting",
      "text": "Community-acquired pneumonia (CAP) develops outside hospital or becomes apparent within the first 48 hours of admission; hospitalization for treatment does not itself make a pneumonia hospital-acquired. NICE’s 2025 definition includes pneumonia developing in nursing-home residents. The usual pathogen spectrum includes Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella, together with respiratory viruses. Their relative frequencies depend on age, geography, season and testing methods. In the US Etiology of Pneumonia in the Community study, published in 2015, a pathogen was identified in 38% of 2,259 hospitalized adults with radiographically confirmed pneumonia and specimens available for bacterial and viral testing. Viruses alone were detected in 23%, bacteria alone in 11%, and both in 3%; pneumococcus was the most frequently identified bacterium. These figures describe hospitalized adults in that study, not all community cases. (nice.org.uk)\n\nHospital-acquired pneumonia (HAP), also called nosocomial pneumonia, is defined in the 2016 American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) guideline as pneumonia beginning at least 48 hours after admission and not incubating on arrival. Ventilator-associated pneumonia (VAP) begins more than 48 hours after endotracheal intubation. That guideline treats HAP as non-ventilator-associated disease and VAP as a separate category, although some publications use HAP as an umbrella term encompassing both. The distinction depends on timing: pneumonia already present when intubation becomes necessary remains community-acquired or hospital-acquired according to its original onset, rather than becoming VAP merely because ventilation is required. Important hospital pathogens include Staphylococcus aureus, Pseudomonas aeruginosa and other Gram-negative bacilli. (idsociety.org)\n\nHealthcare-associated pneumonia (HCAP) was introduced into the 2005 ATS/IDSA framework to identify community-onset cases thought likely to involve resistant organisms. Qualifying exposures included residence in a nursing home or long-term care facility, hospitalization for at least two days during the preceding 90 days, chronic dialysis, home infusion therapy and home wound care. Subsequent evidence showed that this heterogeneous category did not reliably predict resistant pathogens; its use increased broad-spectrum antibiotic treatment without apparent improvement in outcomes. The 2016 HAP/VAP guideline removed HCAP from its scope, and the 2019 CAP guideline explicitly recommended abandoning it as a basis for antibiotic selection. Thus, nursing-home residence alone no longer establishes a separate treatment category in these guidelines. (idsociety.org)\n\nAcquisition setting provides an initial estimate of likely pathogens, but individual exposures and local antibiotic resistance patterns refine that estimate. For community-onset disease, the 2019 guideline identifies previous isolation of methicillin-resistant S. aureus (MRSA) or P. aeruginosa, especially from respiratory specimens, and recent hospitalization with parenteral antibiotics within 90 days as particularly important risk factors. Hospital protocols use local antibiograms—summaries of organisms’ susceptibility to antibiotics—to shape empirical coverage before culture results become available. Setting is therefore not a substitute for resistance assessment: NICE’s 2025 guidance permits community-acquired treatment approaches for selected cases beginning on hospital days 3–5 when resistance risk is low. (doi.org)\n\nAcquisition categories also inform infection-control investigations, but they do not determine isolation precautions by themselves. CDC guidance applies Standard Precautions, including hand hygiene and exposure-appropriate protective equipment, to every patient in every healthcare setting. Additional contact, droplet or airborne precautions depend on the suspected organism and its transmission route. Because the cause may be unknown at presentation, these measures can begin on the basis of the respiratory syndrome and subsequently change when testing identifies the pathogen or excludes a transmissible infection. Community acquisition therefore does not imply an absence of transmission risk within hospital. (cdc.gov)"
    },
    {
      "heading": "Bacterial pneumonia",
      "text": "*Streptococcus pneumoniae*, or pneumococcus, is a leading bacterial cause of community-acquired pneumonia. This encapsulated bacterium commonly colonizes the nose and throat without causing illness; its polysaccharide capsule contributes to virulence by protecting it against phagocytosis. The classical presentation is an abrupt onset of fever, chills, productive cough, breathlessness, and pleuritic chest pain, sometimes with rusty-colored sputum. Chest radiographs classically show lobar consolidation, although patchy bronchopneumonic and multilobar patterns also occur. Neither symptoms nor radiographic distribution reliably identify the organism. Studies of hospitalized patients have differed in the relative frequency of lobar and bronchopneumonic appearances, illustrating the limits of the textbook pattern. (cdc.gov)\n\nThe proportion of pneumonia attributed to pneumococcus depends on the population and detection methods. In the U.S. Etiology of Pneumonia in the Community (EPIC) study, conducted during 2010–2012 and published by Seema Jain and colleagues in 2015, 2,259 hospitalized adults with radiographically confirmed pneumonia underwent testing for both bacterial and viral pathogens. Bacteria were detected in 14%, and pneumococcus—the most commonly detected bacterium—in approximately 5%. These figures describe detected infections rather than the complete etiological distribution: cultures lose sensitivity after antibiotic exposure, and many patients cannot provide suitable sputum. Pneumococcal urinary antigen testing accounted for 67% of pneumococcal detections in that study, demonstrating how additional methods can identify infections missed by culture. (nejm.org)\n\nThe “atypical” bacteria include *Mycoplasma pneumoniae*, Chlamydia pneumoniae, and Legionella species. Their diagnostic requirements, rather than a reliably distinctive clinical appearance, separate them from bacteria readily detected by routine sputum microscopy and culture. M. pneumoniae lacks a cell wall, is not visible by ordinary light microscopy, and requires specialized media with growth taking several weeks; the absence of a cell wall also makes beta-lactam antibiotics ineffective against it. C. pneumoniae likewise requires specialized testing: culture is technically demanding, and serology can require paired samples collected at different stages of illness. Nucleic acid amplification tests, including polymerase chain reaction, are preferred for acute infection with either organism. These bacteria can therefore remain undetected when investigations are limited to routine bacterial culture. (cdc.gov)\n\nLegionnaires’ disease is pneumonia caused by Legionella, most commonly L. pneumophila. Infection usually follows inhalation of contaminated water aerosols or aspiration of contaminated water, linking cases to building water systems, healthcare facilities, and travel accommodation. Routine culture can miss the organism because it requires specialized media. The commonly used urinary antigen test usually detects only L. pneumophila serogroup 1, so a negative result does not exclude other serogroups or species. Lower-respiratory culture or molecular testing broadens detection; culture also permits comparison of patient and environmental isolates during outbreak investigations. “Atypical” does not imply mild disease, and comparative studies have found no radiographic pattern that consistently distinguishes Legionella, mycoplasmal, and pneumococcal pneumonia. (cdc.gov)\n\nIn hospital-acquired and ventilator-associated pneumonia, Staphylococcus aureus and Gram-negative bacilli—including Pseudomonas aeruginosa, Klebsiella pneumoniae, other enteric bacilli, and Acinetobacter baumannii—are prominent causes. Surveillance evidence summarized in the 2016 ATS/IDSA guideline attributed approximately 20–30% of U.S. ventilator-associated pneumonia isolates to S. aureus, 10–20% to P. aeruginosa, and 20–40% to enteric Gram-negative bacilli; these were isolate proportions, not universal patient-level frequencies. Organism distributions vary between hospitals and intensive-care units. Previous respiratory isolation and recent hospitalization with intravenous antibiotics are important indicators of risk for resistant pathogens. S. aureus, including methicillin-resistant strains, can also cause community-acquired pneumonia associated with influenza. (idsociety.org)\n\nMycobacterial lung infections form a separate diagnostic and management category. Pulmonary tuberculosis, caused by the Mycobacterium tuberculosis complex, requires dedicated acid-fast microscopy, molecular testing, and mycobacterial culture, alongside susceptibility testing and tuberculosis-specific infection-control measures. An acid-fast smear cannot distinguish tuberculosis from nontuberculous mycobacteria, and a negative smear does not exclude tuberculosis. Species-specific molecular tests can provide results within hours, whereas culture and identification may take 2–8 weeks. Nontuberculous mycobacteria, including M. avium complex and M. abscessus, usually cause chronic pulmonary disease rather than ordinary acute pneumonia. Because environmental contamination or transient airway presence can produce positive specimens, the 2020 international guideline combines clinical and radiological findings with microbiological criteria, commonly isolation of the same species from at least two separate sputum cultures. Treatment is species-specific and distinct from routine short-course pneumonia therapy. (cdc.gov)"
    },
    {
      "heading": "Viral pneumonia",
      "text": "Influenza A and B viruses, respiratory syncytial virus (RSV), and SARS-CoV-2 can cause primary viral pneumonia, in which viral infection itself produces lung disease rather than merely preceding a bacterial infection. Other associated respiratory viruses include human metapneumovirus, parainfluenza viruses, adenoviruses, rhinoviruses, and seasonal human coronaviruses. Their relative importance differs by age: RSV is particularly prominent in young children, whereas influenza and rhinovirus were the most frequently detected viruses in a major study of hospitalized adults. Severe SARS-CoV-2 pneumonia can involve infection of airway epithelial cells and type II alveolar cells; autopsy studies have documented diffuse alveolar damage, with injury to the lining of the air spaces and formation of hyaline membranes. (nejm.org)\n\nSeasonal patterns influence which viruses are likely to be circulating, but are not diagnostic by themselves. In most of the United States, influenza activity is concentrated in autumn and winter, while RSV circulation typically begins in autumn, peaks in winter, and ends in spring. Human metapneumovirus commonly circulates in late winter and spring, overlapping with both infections. These patterns are geographically variable and can be disrupted: the usual winter RSV epidemic was absent in the United States in 2020–2021, followed by an epidemic beginning in spring 2021. SARS-CoV-2 circulates throughout the year; United States surveillance has recorded summer as well as winter increases, rather than an exclusively winter season. (cdc.gov)\n\nEstimates of the viral share depend on age, study setting, case definitions, and testing methods. The United States Etiology of Pneumonia in the Community (EPIC) studies, led by Seema Jain and published in 2015, examined community-acquired pneumonia requiring hospitalization during 2010–2012. Among 2,222 children with radiographically confirmed pneumonia and adequate specimens, viruses without detected bacteria were identified in 66%, and viral–bacterial combinations in another 7%; RSV was detected in 28%. Among 2,259 adults, the corresponding proportions were 23% and 3%, with rhinovirus detected in 9% and influenza in 6%. These are detection proportions in hospitalized populations, not universal estimates of causation, and they predate SARS-CoV-2. A companion study comparing patients with asymptomatic controls showed why interpretation matters: rhinovirus detection was less clearly associated with pneumonia in children than in adults. A positive upper-airway polymerase chain reaction result therefore does not invariably establish the cause of lung disease. (nejm.org)\n\nBacterial coinfection may be present at initial assessment or develop as secondary superinfection. Influenza-associated injury to respiratory epithelium and changes in antibacterial immune responses facilitate this interaction; experimental work has demonstrated interferon-mediated impairment of bacterial clearance. Pneumococcus and Staphylococcus aureus are important bacterial partners. The 1918–1919 influenza pandemic provides a historical example: in 2008, David Morens, Jeffery Taubenberger, and Anthony Fauci examined lung sections from 58 autopsies and reviewed 109 published autopsy series covering 8,398 investigations. Their findings indicated that most deaths likely resulted directly from secondary bacterial pneumonia following influenza, rather than from isolated viral pneumonia. (pmc.ncbi.nlm.nih.gov)\n\nViral and bacterial pneumonia cannot be reliably separated by bedside presentation alone. Fever, cough, breathlessness, disease severity, and even deterioration after initial improvement require interpretation alongside circulating pathogens and microbiological findings. Detecting influenza does not exclude accompanying bacterial infection. Inflammatory biomarkers also have limitations: the 2019 American Thoracic Society/Infectious Diseases Society of America guideline reported that procalcitonin sensitivity for bacterial infection ranged from 38% to 91% across studies. Neither a low value nor a positive viral test, considered alone, provides a dependable distinction between purely viral disease and viral–bacterial pneumonia. (idsociety.org)"
    },
    {
      "heading": "Fungal, parasitic and noninfectious causes",
      "text": "Opportunistic fungi cause several distinct pulmonary infections when host defenses are impaired. Pneumocystis jirovecii causes predominantly diffuse pneumonia rather than a localized lobar infection. In people with HIV, it typically produces progressive breathlessness, dry cough and fever over days to weeks, with bilateral ground-glass abnormalities on imaging. Approximately 90% of HIV-associated cases occur at CD4 T-cell counts below \\(200\\ \\text{cells/mm}^{3}\\). It also affects people receiving immunosuppressive treatment. Invasive Aspergillus infection follows inhalation of airborne spores; prolonged depletion of granulocytes, hematologic malignancy, transplantation and high-dose corticosteroids are major predisposing conditions. Unlike allergic forms of aspergillosis, invasive disease involves fungal growth into pulmonary tissue and sometimes blood vessels. Cryptococcus neoformans and C. gattii likewise enter through the respiratory tract, producing pneumonia or pulmonary nodules, and may disseminate to the brain and meninges. Most cryptococcal infections occur in immunocompromised hosts, although infection can occur without recognized immunodeficiency. (clinicalinfo.hiv.gov)\n\nEndemic fungal pneumonias depend on environmental exposure and geography rather than exclusively on severe immune impairment. Coccidioides species cause coccidioidomycosis, or Valley fever, most commonly acquired in the southwestern United States. Histoplasma is associated especially with the Ohio and Mississippi River valleys, although its distribution extends well beyond them; inhalation of spores from contaminated soil, including sites enriched with bird or bat droppings, causes infection. Blastomyces provides another example of an endemic soil fungus causing community-acquired pneumonia. Residence in, or travel to, an endemic area can therefore explain pneumonia presenting elsewhere. In surveillance covering 2011–2014, histoplasmosis was reported across multiple US states, illustrating why traditional endemic maps do not define absolute boundaries. These infections can resemble bacterial or viral pneumonia clinically, and symptoms alone do not reliably distinguish histoplasmosis from other respiratory illnesses. (cdc.gov)\n\nParasitic lung disease may result from worms residing in the lungs or from larvae migrating through them. Paragonimus lung flukes are acquired by eating infected raw or undercooked freshwater crabs or crayfish. Larvae penetrate the intestinal wall, cross the diaphragm and enter the lungs, where adult worms generate inflamed cystic cavities; cough, bloody sputum and pleural effusion can resemble tuberculosis. The exposure is not confined to international travel: cases associated with raw crayfish consumption were documented in Missouri during 2006–2010. Strongyloides stercoralis has a different mechanism, maintaining infection through autoinfection within the host. Corticosteroid-associated immune suppression can accelerate this cycle, producing overwhelming larval migration through the gastrointestinal tract and lungs, with pneumonitis, hemorrhage and respiratory failure in hyperinfection syndrome. (cdc.gov)\n\nAspiration pneumonia arises when bacteria-containing oral or gastrointestinal material enters the lower respiratory tract. Swallowing impairment after stroke, neurological disease, sedation, alcohol intoxication and reduced consciousness can compromise airway protection. Small, sometimes unrecognized aspirations of bacteria-rich secretions may be more important than a single witnessed choking episode. The organisms commonly reflect mixed oral flora; anaerobes are not uniformly dominant, although dental disease, putrid sputum and lung abscess increase their relevance. The British Thoracic Society’s 2023 statement distinguishes this infection from aspiration pneumonitis: aspiration of a large volume of acidic vomit initially causes chemical injury rather than bacterial infection. The two processes can overlap if secondary infection develops. (thorax.bmj.com)\n\nNoninfectious pulmonary injury and circulatory disorders can produce pneumonia-like symptoms and radiographic opacities. Inhaled chlorine, for example, injures respiratory tissues and can cause toxic pneumonitis and pulmonary edema; substantial fluid accumulation may be delayed for hours after exposure. Radiation therapy involving the lungs can also provoke inflammatory pneumonitis without an infectious organism. Pulmonary embolism is a vascular mimic rather than an infection: obstruction of pulmonary arteries, sometimes accompanied by infarction, can produce breathlessness, pleuritic pain, fever and focal lung opacities. Pulmonary edema from cardiac dysfunction provides another mimic by filling air spaces with fluid rather than infectious exudate. (cdc.gov)"
    },
    {
      "heading": "Risk factors and host susceptibility",
      "text": "Age strongly influences susceptibility. Children younger than five years have increased risk, particularly infants, whose immune systems are still developing; prematurity further increases vulnerability. At the other extreme, risk rises from age 65 onward rather than changing abruptly at a single birthday. Immunosenescence involves age-related deterioration in both innate and adaptive immune function. Older adults also accumulate chronic illnesses, while diminished cough reflexes weaken airway protection. Frailty is a related but distinct contributor: a 2026 cohort study found that greater frailty and worsening frailty over time were associated with subsequent pneumonia, indicating that chronological age alone does not capture host susceptibility. (cdc.gov)\n\nChronic diseases increase risk through different pathways. Chronic obstructive pulmonary disease, bronchiectasis, cystic fibrosis and asthma are recognized predisposing lung conditions. Heart failure, diabetes and chronic kidney disease also increase susceptibility; advanced kidney disease, particularly nephrotic syndrome and end-stage renal disease, is associated with altered immune function and increased pneumococcal disease risk. Neurological conditions—including stroke, Parkinson’s disease and dementia—can impair swallowing or coughing, allowing saliva, food or gastric material to enter the lungs. These mechanical failures of airway protection differ from primary immune deficiency, although both may coexist in the same patient. (nhlbi.nih.gov)\n\nCancer and its treatment can compromise several components of host defense. Marrow-infiltrating malignancies and cytotoxic chemotherapy may reduce neutrophil numbers; susceptibility depends on both the depth and duration of this reduction. The American Thoracic Society’s 2023 workshop report noted that neutropenia in acute leukemia and before stem-cell transplant engraftment frequently lasts more than ten days, creating substantial vulnerability to pneumonia. Organ and hematopoietic stem-cell transplantation introduce additional risks through immunosuppressive treatment. Long-term systemic corticosteroids suppress immune responses; a 2020 consensus statement used prednisone-equivalent exposure of at least 20 mg daily for at least 14 days as one criterion for identifying immunocompromised patients. This is an operational classification, not a sharp biological boundary between safe and unsafe exposure. (pmc.ncbi.nlm.nih.gov)\n\nThe particular immune defect influences which organisms become important. In HIV infection, bacterial pneumonia risk increases as CD4 T-cell counts decline, while many opportunistic infections become substantially more likely below 200 cells per microliter. Impaired cellular immunity predisposes to organisms such as Pneumocystis jirovecii, whereas severe neutropenia or defective phagocyte function favors invasive mould infections. Absent or dysfunctional splenic tissue creates a different vulnerability: impaired clearance of encapsulated bacteria, especially pneumococci. This occurs after splenectomy and in functional asplenia associated with sickle cell disease. (pmc.ncbi.nlm.nih.gov)\n\nTobacco smoke impairs clearance of airway mucus. A 2019 systematic review covering 27 studies and 460,592 participants found that current smokers had higher odds of community-acquired pneumonia than never-smokers: the pooled odds ratio across 13 studies was 2.17, with a 95% confidence interval of 1.70–2.76. This observational estimate is not an absolute probability of infection. Heavy alcohol exposure adds immune and mechanical risks: it disrupts ciliary activity, alveolar macrophage and neutrophil function, and epithelial barrier integrity; intoxication can also reduce consciousness and facilitate aspiration. (nhlbi.nih.gov)\n\nEnvironmental conditions affect both exposure and host defenses. The World Health Organization identifies household smoke from wood or dung fuels, parental smoking and crowded housing as childhood pneumonia risk factors. Malnutrition weakens immune defenses, particularly in young children, while symptomatic HIV infection and measles add further susceptibility. Crowded settings—including shelters, prisons, military barracks and nursing homes—facilitate exposure to respiratory pathogens. Recent viral illness is another period of vulnerability: bacterial pneumonia can follow a cold or influenza, adding a secondary infection to the preceding respiratory illness. (who.int)"
    },
    {
      "heading": "Clinical presentation",
      "text": "Common symptoms include cough, often with sputum, fever, chills or shaking rigors, shortness of breath, and fatigue. Symptoms may begin abruptly or develop over several days; cough is not invariably productive, and fever is not invariably present. Chest pain, when present, is often pleuritic: sharp pain intensified by deep inspiration or coughing. Lower-lobe infection can cause upper abdominal pain through irritation of the diaphragm, sometimes making the illness appear primarily abdominal. Headache, muscle aches, nausea, vomiting, and diarrhea may accompany the respiratory symptoms. The combination and intensity of symptoms vary substantially, rather than forming a single obligatory clinical pattern. (nhlbi.nih.gov)\n\nIn older adults, an acute change in cognition or function may be more conspicuous than respiratory complaints. Manifestations include new confusion, reduced alertness, weakness, loss of appetite, and falls; temperature may remain normal or fall below normal rather than rise. These features can coexist with cough and breathlessness rather than replace them completely. A 2006 study of 112 older adults admitted with pneumonia illustrates the influence of baseline function: new or increased confusion occurred in 68% of 28 functionally dependent participants, compared with 27% of 84 functionally independent participants. New falls were reported in 29% and 23%, respectively. These figures describe a particular hospitalized cohort, not the frequency in all older people with pneumonia. (nhlbi.nih.gov)\n\nInfants may present with poor feeding, vomiting, irritability, lethargy, or reduced activity, with little obvious cough. Observable respiratory signs include rapid breathing, widening of the nostrils, grunting, and inward movement of the chest wall between the ribs during inspiration. Grunting and chest indrawing indicate increased effort to breathe; severe presentations can include inability to breastfeed or drink, reduced consciousness, and a bluish appearance of the lips or skin. Consequently, an infant’s feeding behavior, alertness, and work of breathing can be more informative than the presence of sputum or a verbal description of chest discomfort. (nhlbi.nih.gov)\n\nPhysical examination may reveal tachypnea, meaning an increased respiratory rate, tachycardia, and localized abnormalities on listening to or percussing the chest. Crackles are brief, discontinuous sounds, usually heard during inspiration, associated with reopening of small airways or air spaces. Consolidated lung may transmit harsh bronchial breath sounds and the spoken voice more clearly than normally aerated lung; egophony describes an “E” sound heard as “A” through the stethoscope. Percussion may produce a dull rather than resonant note over consolidation or pleural fluid. Breath sounds can be diminished over a pleural effusion. These findings are not uniformly present, and crackles also occur in pulmonary edema and other lung disorders. (merckmanuals.com)\n\nThe symptoms overlap with influenza, COVID-19, acute bronchitis, and heart failure, so clinical presentation alone does not establish the underlying cause. Influenza and COVID-19 share fever, cough, breathlessness, fatigue, and muscle aches, and both can themselves cause pneumonia; they are therefore not simply mutually exclusive alternative diagnoses. Acute bronchitis can produce cough and sputum without the pulmonary infiltrate associated with pneumonia. Heart failure is an important noninfectious alternative, particularly when breathlessness and abnormal lung sounds dominate. Differential diagnosis therefore integrates the symptom pattern and examination with imaging and, where relevant, pathogen testing, rather than assigning an organism from symptoms alone. NICE’s 2025 pneumonia guidance explicitly recognizes heart failure as a possible alternative diagnosis when evaluating suspected pneumonia. (cdc.gov)"
    },
    {
      "heading": "Complications and prognosis",
      "text": "Local complications include fluid accumulation around the lung and destruction of infected lung tissue. A pleural effusion associated with pneumonia is termed a parapneumonic effusion; it occupies the space between the lung and chest wall rather than the lung’s air spaces. An uncomplicated effusion may resolve as the infection clears, whereas bacterial invasion can produce a complicated effusion or empyema, a collection of pus in the pleural space. Fibrin deposits can divide infected fluid into separate pockets, making clearance more difficult. The 2023 British Thoracic Society guideline identifies pleural-fluid \\(\\mathrm{pH} \\leq 7.2\\) as indicating a high risk of complicated effusion or pleural infection, generally requiring drainage when safely accessible. A lung abscess differs anatomically: tissue necrosis creates a pus-containing cavity within the lung itself. Abscess formation and more extensive necrotizing pneumonia can account for persistent fever and failure to improve despite treatment. (thorax.bmj.com)\n\nPneumonia can trigger sepsis, in which a dysregulated response to infection causes life-threatening organ dysfunction; bloodstream infection is not required for this process. Septic shock is the subset involving profound circulatory and metabolic abnormalities. Under the adult Sepsis-3 criteria published in 2016, it involves persistent hypotension requiring vasopressors to maintain mean arterial pressure of at least \\(65\\,\\mathrm{mmHg}\\), together with blood lactate above \\(2\\,\\mathrm{mmol/L}\\) despite adequate fluid resuscitation. These criteria identify a particularly high-risk clinical state rather than merely severe fever or inflammation. Pneumonia can also cause acute respiratory distress syndrome (ARDS): damage to the alveolar barrier allows fluid to leak into air spaces, while loss of surfactant promotes alveolar collapse. The resulting impairment of gas exchange can produce profound oxygen deficiency and respiratory failure. Respiratory failure requiring mechanical ventilation and septic shock requiring vasopressors are major criteria for severe community-acquired pneumonia in the 2019 American Thoracic Society–Infectious Diseases Society of America guideline. (doi.org)\n\nExtrapulmonary complications include bacteremia and infection at distant sites. In pneumococcal disease, bacteria entering the bloodstream can spread to the meninges, joints, or heart valves, causing meningitis, septic arthritis, or endocarditis. Cardiac complications are not necessarily caused by direct bacterial spread and are not uniformly rare among hospitalized patients. In a prospective study published in 2012, Vicente F. Corrales-Medina and colleagues followed 1,343 inpatients and 944 outpatients with community-acquired pneumonia for 30 days. New or worsening heart failure, new or worsening arrhythmias, or myocardial infarction occurred in 26.7% of inpatients and 2.1% of outpatients; more than half of the events were recognized within the first 24 hours. Cardiac complications were associated with increased 30-day mortality after adjustment for baseline pneumonia severity. These figures describe that cohort, not a universal complication rate. (cdc.gov)\n\nPoor prognosis is associated with both acute physiological disturbance and limited pre-existing functional reserve. Low oxygenation, hypotension, confusion, multilobar lung involvement, uremia, low platelet or white-cell counts, and hypothermia are markers of severe illness. Frailty adds prognostic information beyond the acute pneumonia episode. A 2021 prospective study of 176 hospitalized adults aged at least 65 years in Seoul found that the combined outcome of death or functional decline within 30 days occurred in 46.7% of the robust group and 86.2% of the severely frail group. The association persisted after adjustment for age, sex, and pneumonia severity. Because the endpoint combined death with increased dependence in daily activities, these percentages are not mortality estimates. (pmc.ncbi.nlm.nih.gov)\n\nRecovery of symptoms and recovery of function proceed on different timescales. NICE’s 2025 guidance describes an expected adult course after treatment in which fever resolves by about one week, chest pain and sputum production substantially diminish by four weeks, and cough and breathlessness substantially diminish by six weeks. Most symptoms resolve by three months, although fatigue may remain; return to normal can take six months. These are approximate trajectories, not fixed deadlines, and recovery varies with severity and underlying health. Frail or chronically ill patients may have persistent disability after the infection has cleared. In a 2022 prospective cohort of 201 older hospitalized patients, frailty at admission was more strongly associated with functional trajectories over six months than pneumonia severity, demonstrating that resolution of infection does not necessarily restore previous independence. (nice.org.uk)"
    },
    {
      "heading": "Diagnosis",
      "text": "Clinical assessment combines the history of acute respiratory illness with examination findings such as increased respiratory rate, focal crackles and altered breath sounds. Recent hospitalization, antibiotic exposure, travel and immune impairment also influence which investigations are appropriate. No single symptom or physical sign reliably establishes pneumonia. Where imaging is unavailable, a clinical diagnosis may be made from the combination of findings; in hospital, chest radiography remains the standard initial imaging test. A new pulmonary opacity supports the diagnosis when accompanied by a compatible illness, but an opacity alone is not specific for infection. Early or subtle pneumonia may not be visible on the first radiograph, so a negative image does not completely exclude disease when clinical suspicion remains high. Radiographic patterns also cannot reliably identify the causative organism. (pmc.ncbi.nlm.nih.gov)\n\nComputed tomography (CT) is more sensitive than radiography and can clarify a negative or indeterminate radiograph in a patient with persistent clinical suspicion. It is particularly useful for suspected abscess, complicated pleural infection or a persistent opacity that may reflect another underlying disorder. Its greater radiation exposure and resource requirements limit routine use. Lung ultrasound provides bedside, radiation-free imaging of peripheral consolidation and pleural fluid, but depends on operator expertise and may miss abnormalities that do not reach the pleural surface. The American Thoracic Society’s 2025 guideline conditionally accepted ultrasound as an alternative to radiography where appropriate expertise exists, citing low-quality evidence. NICE’s September 2025 guideline recognized its value for rapid assessment and pleural complications but retained radiography for confirmation. (acsearch.acr.org)\n\nLaboratory investigations assess physiological disturbance and the inflammatory response rather than independently confirming pneumonia. A complete blood count may show leukocytosis or, in severe infection, leukopenia. Pulse oximetry estimates peripheral oxygen saturation and can reveal hypoxemia despite relatively inconspicuous respiratory symptoms; it does not identify the infection’s cause. C-reactive protein and procalcitonin are inflammatory biomarkers, not definitive bacterial-versus-viral tests. NICE’s 2025 guideline included a baseline C-reactive protein measurement for adults admitted with community-acquired pneumonia. The 2019 ATS/IDSA guideline reported that procalcitonin’s sensitivity for bacterial infection ranged from 38% to 91%, making a low result insufficient to exclude bacterial pneumonia. (pmc.ncbi.nlm.nih.gov)\n\nMicrobiological investigation is selective because test yield, specimen quality and the likelihood of changing management differ between patients. Sputum Gram staining can indicate predominant bacterial forms, while culture can identify organisms and establish antimicrobial susceptibility. However, obtaining a representative lower-airway specimen is often difficult, and prior antibiotics reduce culture yield. The 2019 ATS/IDSA guideline reserved routine sputum and blood cultures principally for severe community-acquired pneumonia and circumstances involving methicillin-resistant Staphylococcus aureus or Pseudomonas aeruginosa risk. Blood-culture positivity in nonsevere adult community-acquired pneumonia was approximately 2–9%; contaminants can also produce misleading results. Endotracheal aspirates provide lower-respiratory samples in intubated patients and generally yield organisms more readily than expectorated sputum. (pmc.ncbi.nlm.nih.gov)\n\nUrinary antigen tests detect components of Streptococcus pneumoniae or Legionella pneumophila without requiring sputum. Their use is concentrated in severe disease, with additional Legionella testing prompted by travel or outbreak exposure. The usual Legionella urinary assay detects mainly L. pneumophila serogroup 1; pairing it with respiratory culture or molecular testing broadens detection. Polymerase chain reaction and other nucleic-acid amplification assays detect respiratory viruses, atypical bacteria and, through multiplex panels, several organisms in one specimen. Their coverage remains limited to the assay’s targets, and results require interpretation alongside imaging and clinical findings. Even extensive testing leaves many cases unexplained: Seema Jain and colleagues’ 2015 US EPIC study identified a pathogen in 853 of 2,259 hospitalized adults with radiographically confirmed pneumonia and both bacterial and viral testing—38%. Limited access to lower-airway specimens, preceding antibiotics and imperfect test sensitivity contributed to the diagnostic gap. (idsociety.org)"
    },
    {
      "heading": "Severity assessment and site of care",
      "text": "CURB-65 and the Pneumonia Severity Index (PSI) estimate 30-day mortality in adults with community-acquired pneumonia; they support, rather than replace, clinical decisions about hospitalization. CURB-65, developed by W. S. Lim and colleagues in 2003, assigns one point each for confusion, blood urea above \\(7\\,\\mathrm{mmol/L}\\), respiratory rate at least 30 breaths/minute, systolic blood pressure below \\(90\\,\\mathrm{mmHg}\\) or diastolic pressure at or below \\(60\\,\\mathrm{mmHg}\\), and age at least 65 years. The resulting score ranges from 0 to 5. NICE guidance groups scores of 0–1 as low risk, 2 as intermediate risk, and 3–5 as high risk, with estimated mortality below 3%, 3–15%, and above 15%, respectively. These are prognostic categories, not fixed probabilities for every population. (pubmed.ncbi.nlm.nih.gov)\n\nMichael J. Fine and colleagues published the PSI in 1997. It combines demographic characteristics, coexisting illnesses, examination findings, and laboratory and radiographic measurements to assign five risk classes. In its original cohorts, mortality ranged from 0.1–0.4% in class I, 0.6–0.7% in class II, and 0.9–2.8% in class III. The 2019 American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) guideline preferred PSI over CURB-65 for hospitalization decisions because PSI-guided pathways had stronger evidence of safely increasing outpatient treatment. Nevertheless, PSI can underestimate severity in younger patients. Neither score was designed to determine intensive-care requirements, and both were developed principally in immunocompetent adults rather than children or severely immunocompromised populations. (nejm.org)\n\nHospital assessment also considers abnormalities incompletely represented by mortality scores: low oxygen saturation, inability to maintain oral intake, substantial comorbidity, cognitive impairment, and poor functional status. NICE identifies room-air oxygen saturation below 90%, or failure to meet a patient’s established oxygen requirement, as clinical instability; suspected cardiorespiratory failure or sepsis warrants hospital referral. Consequently, a low CURB-65 score does not exclude a need for admission when oxygenation is impaired. Conversely, chronological age alone does not establish a need for intensive care. The choice of setting incorporates physiological findings, practical support, and the patient’s preferences. (nice.org.uk)\n\nIntensive-care assessment uses organ-support requirements and separate severe-pneumonia criteria. ATS/IDSA recommends direct intensive-care admission for shock requiring vasopressors or respiratory failure requiring mechanical ventilation. Its severe community-acquired pneumonia definition comprises either one of these major criteria or at least three minor criteria: respiratory rate at least 30/minute; arterial oxygen pressure to inspired oxygen fraction ratio \\(P_{\\mathrm{a}O_2}/F_{\\mathrm{i}O_2}\\leq250\\); multilobar infiltrates; confusion; blood urea nitrogen at least \\(20\\,\\mathrm{mg/dL}\\); leukopenia below \\(4{,}000/\\mu\\mathrm{L}\\); platelets below \\(100{,}000/\\mu\\mathrm{L}\\); core temperature below \\(36^\\circ\\mathrm{C}\\); or hypotension requiring aggressive fluid resuscitation. The minor criteria guide higher-intensity monitoring alongside clinical judgment, rather than automatically determining placement. (pmc.ncbi.nlm.nih.gov)\n\nOutpatient care depends on more than a low predicted mortality: patients must be able to take oral treatment and have adequate functional and social support. NICE’s 2025 guidance allows primary-care management with safety-netting for low-risk adults, while selected intermediate-risk patients may receive hospital-at-home, virtual-ward, or same-day emergency care. Safety-netting specifies when reassessment is needed and how it can be obtained. NICE identifies rapid or substantial deterioration, becoming systemically unwell, or failure to begin improving within three days as reasons for further assessment; these instructions do not imply that complete symptom resolution should occur within three days. (pmc.ncbi.nlm.nih.gov)\n\nWhere laboratory testing is unavailable, CRB-65 omits the urea measurement and uses confusion, respiratory rate, blood pressure, and age. NICE associates scores of 0, 1–2, and 3–4 with mortality risks below 1%, 1–10%, and above 10%. For young children, the World Health Organization instead uses clinical assessment through Integrated Management of Childhood Illness: inability to drink, vomiting everything, convulsions, and lethargy or unconsciousness are danger signs informing referral. These pathways permit decisions without radiography or blood tests, but clinical recognition of hypoxemia remains imperfect. WHO’s 2016 oxygen manual therefore emphasizes pulse oximetry as a more reliable means of detecting low oxygen saturation in resource-limited facilities. (nice.org.uk)"
    },
    {
      "heading": "Antimicrobial treatment",
      "text": "Empirical antibiotic treatment targets likely pathogens before microbiological results are available; its breadth depends on acquisition setting, severity, previous isolates and local resistance patterns. The 2019 American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) adult community-acquired pneumonia guideline lists amoxicillin or doxycycline for otherwise healthy outpatients without resistant-pathogen risk factors. Macrolide monotherapy is restricted to settings where local pneumococcal macrolide resistance is below 25%. For outpatients with specified comorbidities, options include amoxicillin–clavulanate or a cephalosporin combined with a macrolide or doxycycline, or a respiratory fluoroquinolone alone. These are adult recommendations, not interchangeable pediatric regimens. (pmc.ncbi.nlm.nih.gov)\n\nFor hospitalized adults with nonsevere community-acquired disease, standard regimens include a beta-lactam plus a macrolide, such as ceftriaxone plus azithromycin, or respiratory fluoroquinolone monotherapy, such as levofloxacin or moxifloxacin. Severe disease requires a beta-lactam combined with either a macrolide or a respiratory fluoroquinolone; fluoroquinolone monotherapy is not a recommended severe-disease regimen. Combination treatment extends coverage to organisms such as Legionella and Mycoplasma, which are not reliably treated by beta-lactams. Drug selection also accounts for allergies, renal function, interactions and adverse-effect risks. (pmc.ncbi.nlm.nih.gov)\n\nHospital-acquired and ventilator-associated pneumonia require different empirical coverage. The 2016 ATS/IDSA guideline uses hospital—and, where available, intensive-care-unit—antibiograms to select activity against Staphylococcus aureus, Pseudomonas aeruginosa and other Gram-negative bacilli. Examples of antipseudomonal agents include piperacillin–tazobactam, cefepime and meropenem. Vancomycin or linezolid provides methicillin-resistant S. aureus coverage when indicated. Previous intravenous antibiotics within 90 days, local resistance frequencies and high mortality risk influence whether two antipseudomonal drugs from different classes are initially used. Broad combination treatment is therefore conditional rather than routine for every hospitalized patient. (idsociety.org)\n\nCulture and susceptibility results permit de-escalation: replacing broad empirical coverage with a narrower active drug, or reducing combination treatment to monotherapy. The 2016 guideline generally specifies seven days for hospital-acquired or ventilator-associated pneumonia, adjusted to clinical response. (idsociety.org) For community-acquired disease, the 2025 ATS update conditionally supports fewer than five days, with a minimum of three, in immunocompetent adults with nonsevere disease who reach clinical stability; severe disease requires at least five days. Stability includes improvement in vital signs and oxygenation, normal mentation and ability to eat. Abscess, empyema, bacteremia, necrotizing infection or particular pathogens can require longer treatment. (doi.org) Supporting evidence includes Aurélien Dinh and colleagues’ 2021 randomized trial: among 303 hospitalized adults stable after three days of beta-lactam treatment, day-15 cure occurred in 77% assigned to stop treatment and 68% assigned five additional days, meeting the prespecified noninferiority criterion. This does not establish that three days suffice for unstable or critically ill patients. (pubmed.ncbi.nlm.nih.gov)\n\nAntibiotics do not treat viral infection itself, but a positive viral test does not exclude bacterial coinfection. The 2025 ATS guideline conditionally suggests withholding empirical antibiotics in otherwise healthy adult outpatients with clinical and imaging evidence of pneumonia and a positive respiratory-virus test. It favors antibiotics for virus-positive outpatients with comorbidities and for hospitalized patients, reflecting the consequences of missed bacterial infection. These recommendations have very low-quality supporting evidence and require assessment of the whole presentation rather than the test result alone. (doi.org)\n\nAntiviral treatment is pathogen-specific. The 2018 IDSA influenza guideline recommends prompt treatment, usually with oseltamivir, for hospitalized influenza patients regardless of illness duration before admission; benefit can persist when treatment begins more than 48 hours after symptom onset. Severe influenza pneumonia may require simultaneous antibacterial treatment for coinfection. (idsociety.org) For eligible high-risk patients with mild or moderate COVID-19, early options include nirmatrelvir–ritonavir within five days of symptom onset or a three-day intravenous remdesivir course started within seven days. Nirmatrelvir–ritonavir requires screening for important drug interactions. (cdc.gov) In hospitalized severe COVID-19, IDSA supports remdesivir in selected patients, but advises against its routine initiation after invasive ventilation or extracorporeal membrane oxygenation has begun. (idsociety.org)"
    },
    {
      "heading": "Supportive and intensive care",
      "text": "Supplemental oxygen treats hypoxemia rather than the infection itself, and its delivery is adjusted to measured oxygen saturation. The British Thoracic Society’s 2017 guideline specifies a target saturation of 94–98% for adults with pneumonia who are not at risk of hypercapnic respiratory failure, and generally 88–92% for those at risk of carbon dioxide retention, pending blood-gas assessment. These are guideline-specific ranges, not a requirement to maximize saturation. Nasal cannulae or face masks provide initial support; persistent hypoxemia may require higher-flow delivery and closer monitoring. (doi.org)\n\nHigh-flow nasal oxygen supplies heated, humidified gas at flows that better match inspiratory demand. Noninvasive ventilation delivers positive pressure through a mask or helmet, supporting ventilation without an endotracheal tube. The 2023 European severe community-acquired pneumonia guideline conditionally favors high-flow nasal oxygen over conventional oxygen in hypoxemic patients not requiring immediate intubation, while allowing noninvasive ventilation in selected patients, particularly when hypoventilation or increased breathing effort is prominent. Both approaches require frequent reassessment: worsening respiratory distress, impaired consciousness, or persistent gas-exchange failure can make invasive mechanical ventilation necessary, and unsuccessful noninvasive support must not delay intubation. (elibrary.escmid.org)\n\nCorticosteroids are adjuncts intended to suppress excessive inflammation, not substitutes for antimicrobial treatment. In the 2023 CAPE COD randomized controlled trial, Pierre-François Dequin and colleagues analyzed 795 adults in intensive care with severe community-acquired pneumonia. Intravenous hydrocortisone, initially 200 mg daily followed by tapering, reduced 28-day mortality from 11.9% with placebo to 6.2%; treated patients required more insulin during the first week. (pubmed.ncbi.nlm.nih.gov) Evidence is not uniform: the 2025 REMAP-CAP trial stopped enrollment to its fixed seven-day hydrocortisone regimen for futility and did not establish improved 90-day survival. (pubmed.ncbi.nlm.nih.gov) The 2025 American Thoracic Society guideline nevertheless conditionally supports systemic corticosteroids for severe adult community-acquired pneumonia, with low-quality evidence, but recommends against their use in nonsevere disease; its severe-pneumonia recommendation excludes influenza pneumonia. (doi.org)\n\nPneumonia-associated septic shock requires restoration of tissue perfusion while avoiding fluid accumulation that can worsen respiratory failure. The 2021 Surviving Sepsis Campaign suggests at least 30 mL/kg of intravenous crystalloid within three hours for sepsis-induced hypoperfusion or septic shock—a weak recommendation based on low-quality evidence. Subsequent fluid administration is guided by repeated assessment, preferably including dynamic measures such as the response to passive leg raising, rather than continued automatic boluses. Balanced crystalloids are favored over saline. Norepinephrine is the first-line vasopressor, with an initial mean arterial pressure target of 65 mmHg. The associated care bundle combines lactate measurement and reassessment, blood cultures before antimicrobials when this causes no substantial delay, prompt antimicrobial administration, fluid resuscitation, and vasopressors for persistent hypotension. (pmc.ncbi.nlm.nih.gov)\n\nWhen pneumonia produces acute respiratory distress syndrome (ARDS), invasive ventilation uses lung-protective settings to limit additional injury from excessive distension. These include tidal volumes around 6 mL/kg of predicted—not actual—body weight, plateau pressure no higher than approximately 30 cm of water, and positive end-expiratory pressure to maintain lung recruitment. (sccm.org) In the ARDS Network’s 2000 trial of 861 patients, lower tidal volumes reduced mortality from 39.8% to 31.0% compared with traditional larger volumes. (doi.org) For severe ARDS, prolonged prone positioning is another evidence-based intervention: the 2013 PROSEVA trial used sessions lasting at least 16 hours and reported 28-day mortality of 16.0%, versus 32.8% with supine positioning. These trials enrolled ARDS populations rather than pneumonia patients exclusively. (nejm.org)\n\nFor mild disease managed at home, supportive care commonly includes sleep and rest, oral fluids to maintain hydration, and medicines for fever or muscle pain when clinically appropriate. These measures relieve symptoms but do not replace treatment directed at the cause. Cough and cold preparations are not routine components of every treatment plan; their use is assessed separately by the treating clinician. Recovery guidance also emphasizes adequate nutrition and avoidance of tobacco smoke, which can worsen pulmonary symptoms. (nhlbi.nih.gov)"
    },
    {
      "heading": "Pneumonia in children and older adults",
      "text": "In low-resource settings, childhood pneumonia is commonly classified using observable signs rather than radiography or microbiological testing. The World Health Organization (WHO) algorithm identifies pneumonia in children aged 2–59 months with cough or difficult breathing when fast breathing or lower chest-wall indrawing is present. Fast breathing means at least 50 breaths per minute at 2–11 months and at least 40 at 12–59 months; indrawing means that the lower chest wall retracts during inspiration. These are operational criteria for treatment decisions, not proof of bacterial infection. Inability to drink or breastfeed, convulsions, lethargy or unconsciousness indicate greater severity. WHO’s 2024 guideline permits outpatient treatment of chest-indrawing pneumonia without general danger signs, and conditionally supports community-based treatment where trained health workers, medicines and follow-up are available. (paho.org)\n\nHypoxemia and malnutrition are particularly important determinants of childhood deaths. Alveolar fluid and inflammatory material limit oxygen uptake, while undernutrition weakens immune defenses. Pulse oximetry detects low blood oxygen that may not be apparent from respiratory signs alone; WHO uses an oxygen saturation below 90% as a marker of severe disease. Access depends not only on oxygen supplies but also on functioning equipment and systems for recognizing hypoxemia. In a five-hospital Papua New Guinea study published in 2008, Trevor Duke and colleagues introduced oxygen concentrators, pulse oximeters and treatment protocols. Among more than 11,000 pediatric pneumonia admissions, case fatality fell from 4.97% before implementation to 3.22% afterward—a 35% relative reduction. Because this was a before-and-after study, the result reflects the combined service intervention rather than an isolated oxygen effect. (who.int)\n\nPediatric antibiotic use involves a tension between treating potentially dangerous bacterial disease and exposing children with self-limiting illness to unnecessary medication. A double-blind randomized controlled trial published in 2018 enrolled 1,126 HIV-uninfected Malawian children aged 2–59 months with nonsevere fast-breathing pneumonia. Treatment failure by day four occurred in 4.0% receiving amoxicillin and 7.0% receiving placebo; approximately 33 children needed treatment to prevent one failure. Nevertheless, about 93% of placebo recipients avoided early failure, and neither group had deaths. The trial therefore demonstrated both a measurable antibiotic benefit and the limited specificity of syndromic classification for antibiotic-responsive illness. WHO’s 2024 guideline retains oral amoxicillin for fast-breathing pneumonia, balancing this evidence against the consequences of withholding treatment where diagnostic testing and reassessment are limited. (pubmed.ncbi.nlm.nih.gov)\n\nIn older adults, frailty—reduced physiological reserve and vulnerability to acute stress—affects outcomes beyond chronological age. Pneumonia may present with delirium, reduced food intake or loss of everyday function rather than prominent fever and cough. Swallowing impairment and dependence on others for feeding also increase the importance of aspiration. A prospective study of 189 older patients followed for up to four years found that swallowing difficulty was an important risk factor, but generally insufficient by itself to produce aspiration pneumonia; feeding dependence, oral-care dependence and dental disease were among the additional predictors. Thus, aspiration-related infection reflects interacting functional and oral-health problems rather than simply an observed choking episode. (pmc.ncbi.nlm.nih.gov)\n\nFunctional recovery can remain incomplete after the acute infection resolves. In a prospective Seoul hospital cohort published in 2021, 176 pneumonia patients aged at least 65 years had a median age of 79. After adjustment for age, sex and pneumonia severity, the estimated probability of death or functional decline within 30 days rose from 46.7% in robust patients to 86.2% in severely frail patients. Greater frailty was also associated with prolonged hospitalization and discharge to long-term care. These outcomes capture losses of independence that mortality statistics alone omit. (pmc.ncbi.nlm.nih.gov)\n\nLong-term care facilities also face outbreaks of respiratory viruses, including influenza, respiratory syncytial virus and SARS-CoV-2, which can cause pneumonia. Residents may lack fever or recognizable respiratory symptoms, complicating early detection. CDC’s nursing-home respiratory-virus toolkit consequently addresses rapid testing, separation of affected residents, transmission-based precautions and outbreak investigation. These measures concern transmissible respiratory pathogens; aspiration-related pneumonia does not itself imply a contagious facility outbreak. (cdc.gov)"
    },
    {
      "heading": "Prevention and vaccination",
      "text": "Pneumococcal vaccines target selected capsular serotypes of Streptococcus pneumoniae, rather than every cause of pneumonia. Conjugate vaccines (PCVs) attach bacterial polysaccharides to a carrier protein, producing a T-cell-dependent response that is effective in infancy and supports immunological memory. The unconjugated 23-valent polysaccharide vaccine, PPSV23, induces protective antibodies but responds poorly in children younger than two years. Its protection is better established against invasive pneumococcal disease than against nonbacteremic pneumonia. Childhood PCV programs also reduce carriage and transmission of vaccine-type strains, indirectly protecting unvaccinated people. CDC surveillance documented a 90% decline in invasive disease caused by PCV13 serotypes among children after its introduction in 2010; this is not a measure of the reduction in pneumonia from all causes. (cdc.gov)\n\nDirect protection against adult pneumonia was demonstrated in the CAPiTA randomized controlled trial, reported by Marc Bonten and colleagues in 2015. Among 84,496 adults aged at least 65 years in the Netherlands, PCV13 efficacy was 45.6% against first episodes of vaccine-type pneumococcal community-acquired pneumonia and 75.0% against vaccine-type invasive disease. (pubmed.ncbi.nlm.nih.gov) Vaccination schedules differ by country and previous immunization. Under CDC guidance published in February 2026, routine childhood vaccination uses PCV15 or PCV20 at 2, 4, 6, and 12–15 months. Adults who have never received a PCV are eligible at age 50 or older, or at ages 19–49 with specified risk conditions. Adult options include PCV20 or PCV21 alone, or PCV15 followed by PPSV23; previous doses determine subsequent requirements. (cdc.gov)\n\nOther vaccines prevent particular bacterial or viral causes. Haemophilus influenzae type b (Hib) conjugate vaccination is routinely directed at children younger than five; selected older recipients include people with absent or dysfunctional spleens and hematopoietic stem-cell transplant recipients. Hib vaccination does not protect against every strain of H. influenzae. (cdc.gov) Influenza vaccination reduces influenza and its complications; CDC guidance specifies annual vaccination from six months of age, with particular importance for older adults, pregnant people, and people with chronic disease. (cdc.gov) COVID-19 vaccines chiefly protect against severe disease and death. WHO guidance in September 2026 prioritized older adults, people with substantial comorbidities or immunocompromise, and long-term-care residents, with additional vaccination groups including pregnant people and healthcare workers caring directly for high-risk patients. National eligibility and revaccination intervals vary. (who.int)\n\nRespiratory syncytial virus (RSV) prevention includes both active vaccination and passive antibody protection. CDC guidance in February 2026 recommended a single vaccine dose for all adults aged 75 or older and adults aged 50–74 at increased risk of severe RSV disease; it was not an annual vaccination. (cdc.gov) Maternal vaccination generates antibodies that cross the placenta and protect infants during early life. In most of the continental United States, the maternal vaccine is administered at 32–36 weeks of pregnancy during September–January. Alternatively, eligible infants receive a long-acting monoclonal antibody, such as nirsevimab or clesrovimab: these products provide passive protection and are not vaccines. Most infants do not require both approaches. (cdc.gov)\n\nTransmission control includes handwashing with soap, covering coughs and sneezes, disposing of tissues, and cleaning hands after contact with respiratory secretions. Healthcare facilities combine these measures with early recognition of respiratory symptoms, masks for source control, appropriate patient placement, pathogen-specific precautions, and cleaning of shared equipment. (who.int) Hospital pneumonia prevention also addresses aspiration and invasive devices. The 2022 hospital-prevention guidance includes regular oral care, assessment and management of swallowing difficulties, and measures that reduce unnecessary ventilation. Swallowing assessment can lead to supervised feeding, adjusted food or liquid consistency, and upright positioning during meals. (pmc.ncbi.nlm.nih.gov)\n\nReducing tobacco exposure and household air pollution addresses preventable risks beyond vaccination. WHO identifies smoking cessation and replacement of polluting household cooking arrangements with cleaner fuels and technologies as pneumonia-prevention measures. Adequate nutrition supports host defenses, particularly where malnutrition increases childhood susceptibility. Exclusive breastfeeding during the first six months, followed by nutritionally adequate complementary feeding with continued breastfeeding, is part of the WHO–UNICEF prevention framework. (who.int)"
    },
    {
      "heading": "Global burden and epidemiology",
      "text": "Global estimates usually measure the broader category of lower respiratory infections (LRIs), rather than clinically confirmed pneumonia alone. The Global Burden of Disease Study (GBD) combines vital-registration records, verbal autopsies, surveillance and other data with statistical models to estimate deaths where reporting is incomplete. Its LRI category excludes tuberculosis, pertussis and COVID-19, which are counted separately. Consequently, these figures are not totals for every disease that can produce pneumonia, and their uncertainty intervals reflect limitations in both data and modelling. (pmc.ncbi.nlm.nih.gov)\n\nThe GBD 2023 analysis, published online on December 15, 2025, estimated 2.50 million LRI deaths worldwide in 2023, with a 95% uncertainty interval of 2.24–2.81 million. It estimated 242 million incident episodes and an all-age mortality rate of 31.0 deaths per 100,000 person-years. Within the same modelled time series, deaths increased from 2.12 million in 2021 to 2.50 million in 2023—approximately 18%—after declining during 2020–2021. This increase should be distinguished from comparisons between successive GBD publications, which also revise earlier estimates. (discovery.ucl.ac.uk)\n\nDeaths were concentrated at the extremes of age. In 2023, children younger than five accounted for approximately 610,000 deaths, while adults aged 70 and older accounted for 1.25 million; together, these groups represented about three-quarters of LRI deaths. Their respective mortality rates were 94.8 and 231.6 per 100,000 person-years. From 2010 to 2023, under-five mortality rates fell by 33.4%, whereas the estimated decline among adults aged 70 and older was only 10.2%, with an uncertainty interval that included no change. (discovery.ucl.ac.uk)\n\nGeographic comparisons distinguish large death counts from high population-adjusted rates. A GBD 2019 analysis estimated approximately 548,000 deaths in South Asia, 677,000 across the four sub-Saharan African regions, and 205,000 in East Asia: together, roughly 57% of the global total. East Asia’s age-standardized mortality rate was nevertheless substantially lower than those in sub-Saharan Africa. (frontiersin.org) Socioeconomic inequalities were also pronounced: GBD 2021 estimated under-five mortality rates of 153.2 per 100,000 in countries in the lowest Socio-demographic Index quintile, compared with 1.9 in the highest—a difference of about eightyfold. (pmc.ncbi.nlm.nih.gov)\n\nLong-term trends differ according to the measure used. Age-standardization applies a common age distribution, allowing comparisons that are less affected by population ageing. The GBD 2019 analysis estimated a 48.5% reduction in global age-standardized LRI mortality between 1990 and 2019, including declines of 76.2% in East Asia and 50.4% in South Asia. Eastern Europe showed little net change, with an estimated increase of 1.0% whose uncertainty interval included a decline. (frontiersin.org) Falling rates did not necessarily produce falling death counts: a separate GBD 2019 analysis estimated that deaths among men aged 70 and older doubled between 1990 and 2019, despite a 19.1% decline in their mortality rate, illustrating the effects of demographic growth and ageing. (pmc.ncbi.nlm.nih.gov)"
    },
    {
      "heading": "Antimicrobial resistance and pathogen evolution",
      "text": "Antimicrobial resistance in Streptococcus pneumoniae arises through several distinct mechanisms. Reduced susceptibility to penicillin and other β-lactams primarily reflects altered penicillin-binding proteins, which reduce drug binding to enzymes involved in bacterial cell-wall synthesis. Macrolide resistance commonly involves erm-mediated modification of the ribosomal target or mef-mediated drug efflux. These mechanisms can coexist in multidrug-resistant lineages. Resistance affects empirical treatment—the initial regimen chosen before the pathogen and its susceptibility are known. The 2019 American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) guideline restricted macrolide monotherapy for otherwise healthy adult outpatients to settings where local pneumococcal macrolide resistance was below 25%. Penicillin resistance figures also require attention to laboratory definitions: in 2008, susceptibility breakpoints were revised for intravenous penicillin treatment of non-meningeal pneumococcal infections, so historical percentages cannot be compared without accounting for this change. (wwwnc.cdc.gov)\n\nIn hospital-acquired and ventilator-associated pneumonia, resistance concerns extend to Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA). K. pneumoniae can produce extended-spectrum β-lactamases, which inactivate most penicillins and cephalosporins, or carbapenemases such as KPC, NDM, and OXA-48-like enzymes. Additional mechanisms, including reduced outer-membrane permeability, can further restrict treatment options. Identifying the enzyme matters because newer β-lactam/β-lactamase-inhibitor combinations do not cover all carbapenemases equally. For MRSA pneumonia, the 2016 ATS/IDSA hospital-pneumonia guideline identified vancomycin and linezolid as the principal treatment options. The same guideline emphasized hospital- or intensive-care-unit-specific susceptibility summaries, rather than assuming that every hospital has the same pathogen distribution or resistance prevalence. (idsociety.org)\n\nPrior antibiotic exposure selects resistant organisms by suppressing susceptible competitors and is an important predictor of resistant pneumonia. In the evidence synthesis supporting the 2016 hospital-pneumonia guideline, intravenous antibiotic use during the preceding 90 days was associated with multidrug-resistant ventilator-associated pneumonia with an odds ratio of 12.3 (95% confidence interval, 6.48–23.35); these observational associations do not establish an individual patient’s absolute risk. Antimicrobial stewardship programs address unnecessary selection pressure through restricted-drug authorization, prospective prescription review with feedback, susceptibility-guided narrowing, and avoidance of unnecessarily prolonged courses. Their purpose is not simply to minimize antibiotic use, but to retain effective treatment while reducing avoidable exposure. The 2016 IDSA/Society for Healthcare Epidemiology of America stewardship guideline identified preauthorization and prospective audit with feedback as core interventions. (idsociety.org)\n\nPneumococcal vaccination changes the population in which resistance evolves. Conjugate vaccines target selected capsular serotypes and reduce disease caused by those types, including resistant strains. After the seven-valent vaccine, PCV7, entered US use in 2000, disease caused by covered resistant serotypes declined, while non-covered serotype 19A became more prominent. PCV13, introduced in 2010, included 19A. Between 2009 and 2013, antibiotic-resistant invasive pneumococcal disease caused by the additional PCV13 serotypes declined by 97% among children younger than five and 64% among adults aged 65 or older. These are invasive-disease surveillance estimates, not reductions measured across all pneumonia episodes. Serotype replacement therefore can partly offset vaccine gains without negating the substantial reduction in targeted disease. (cdc.gov)\n\nReplacement also differs from capsular switching: genetic recombination can give an existing pneumococcal lineage a different capsule while retaining resistance-associated characteristics. A US genomic study of isolates collected in 2015–2016 documented an emerging penicillin-nonsusceptible 35B/ST156 lineage arising through a single ancestral recombination event. Serotype 35B was not included in PCV13. Such findings make surveillance of both capsule type and genetic lineage relevant to vaccine design: the number of included serotypes alone does not describe coverage of circulating resistant strains, and expanded formulations require continued monitoring of disease caused by both covered and non-covered types. (wwwnc.cdc.gov)"
    },
    {
      "heading": "Historical understanding and treatment",
      "text": "Greek medical writings associated with Hippocrates, who lived approximately 460–370 BCE, described an acute illness called peripneumonia, characterized by fever, chest pain and cough. These descriptions recorded a recognizable clinical syndrome rather than identifying a microbial cause. The separation between recognizing pneumonia and understanding its origin persisted for more than two millennia. Even nineteenth-century treatment included bloodletting: Pierre-Charles-Alexandre Louis used numerical comparisons of pneumonia cases to question its effectiveness, an early example of evaluating treatment through systematically collected clinical observations. (library.leeds.ac.uk)\n\nThe development of germ theory in the late nineteenth century transformed pneumonia from a syndrome classified principally by symptoms and lung pathology into an infection whose causes could be investigated experimentally. In 1875, Edwin Klebs observed bacteria in the airways of pneumonia patients. In 1881, George Miller Sternberg and Louis Pasteur independently isolated the organism subsequently named Streptococcus pneumoniae, or pneumococcus, and demonstrated its pathogenic potential by inoculating rabbits. Work by Carl Friedländer and Albert Fraenkel during the following years helped connect bacterial findings with human pneumonia. The introduction of Gram staining in 1884 improved the differentiation of organisms that had previously been confused, while the recognition of numerous pneumococcal serotypes showed that apparently similar infections could involve immunologically distinct bacteria. (library.leeds.ac.uk)\n\nBefore effective antibiotics, pathogen-specific serum therapy offered a treatment for some pneumococcal infections. At the Rockefeller Hospital, Rufus Cole and Alphonse Dochez prepared antiserum from horses inoculated with bacteria isolated from pneumonia patients. By the end of 1912, they had developed a serum against type I pneumococcus and a method for determining whether a patient carried that type. Treatment supplied ready-made antibodies, rather than inducing the patient to produce them, and depended on matching the serum to the infecting organism. Cole reported this approach in 1913. Its specificity made laboratory typing integral to treatment; during the First World War, United States Army and Navy medical officers trained at Rockefeller in pneumococcal identification and selective serum administration. (centennial.rucares.org)\n\nSulfonamides changed treatment in the late 1930s by providing an effective drug treatment that did not require a matching antiserum. In 1938, Lionel Whitby reported experimental protection against pneumococcal infection using sulfapyridine, initially designated M&B 693. That July, G. M. Evans and Wilfrid Gaisford reported 8 deaths among 100 pneumonia patients treated with the compound, compared with 27 deaths among 100 controls receiving nonspecific treatment. Penicillin subsequently expanded the treatment options. Alexander Fleming discovered its antibacterial activity in 1928; Howard Florey, Ernst Chain and their collaborators developed it into a practical medicine in the early 1940s. Industrial fermentation made widespread use possible: United States production rose from 21 billion units in 1943 to 1,663 billion in 1944 and more than 6.8 trillion in 1945. (mdedge.com)\n\nPneumococcal vaccine development began as early as 1911, but interest diminished after penicillin became available and revived when deaths continued despite antibiotic treatment. United States licensure of a 14-valent capsular-polysaccharide vaccine in 1977 was followed by a 23-valent formulation in 1983. Conjugation of capsular sugars to a protein carrier subsequently made effective infant vaccination possible: seven-valent PCV7 was licensed in 2000 and thirteen-valent PCV13 in 2010. Among United States children younger than five, invasive disease caused by PCV7 serotypes fell from 80.0 cases per 100,000 in 1998–1999 to 4.6 in 2003, a 94% reduction; these figures concern invasive pneumococcal disease, not all pneumonia. Related vaccine developments also changed childhood respiratory disease: invasive Haemophilus influenzae type b disease declined by 99% between 1989 and 1995 as conjugate vaccination expanded. (cdc.gov)"
    },
    {
      "heading": "Research frontiers and unresolved questions",
      "text": "Rapid molecular diagnostics are being evaluated for their ability to change treatment, rather than merely detect more organisms. In a 2022 randomized controlled trial involving 200 critically ill adults with pneumonia, molecular point-of-care testing returned results in a median of 1.7 hours, compared with 66.7 hours for conventional testing. Antibiotics were de-escalated—stopped or replaced with narrower-spectrum agents—in 42% of the molecular-testing group and 8% of controls. However, total antibiotic duration and clinical outcomes did not differ substantially. This distinction remains important: faster identification can improve prescribing without necessarily reducing mortality or treatment length. (pubmed.ncbi.nlm.nih.gov) The unresolved etiological gap is substantial. In Seema Jain and colleagues’ 2015 EPIC study, comprehensive testing identified a pathogen in only 38% of 2,259 hospitalized adults with radiographically confirmed pneumonia. The investigators identified limitations including prior antibiotic exposure, difficulty obtaining lower-airway specimens, and incomplete diagnostic sensitivity. (nejm.org)\n\nHost-response biomarkers offer a complementary approach by measuring the response to infection rather than searching directly for a microorganism. The Apollo study, published in 2024, evaluated a test combining tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), interferon-γ-inducible protein 10 (IP-10), and C-reactive protein. TRAIL and IP-10 contribute information about antiviral immune responses, while C-reactive protein contributes an inflammatory signal. Among 476 evaluable adults and children, the test achieved 90.0% sensitivity and 92.8% specificity for bacterial infection against expert-adjudicated diagnoses; 7.2% of results were equivocal. Only 11.3% of participants had lower respiratory tract infections, however, and clinicians did not receive the results. These findings establish diagnostic performance in a mixed acute-infection population, not proof that biomarker-guided pneumonia treatment safely reduces antibiotic use. (pmc.ncbi.nlm.nih.gov)\n\nResearch on shorter courses increasingly selects patients by clinical recovery. Aurélien Dinh and colleagues’ 2021 PTC trial found that stopping β-lactam therapy after three days was noninferior to continuing for eight days in hospitalized adults outside critical care who had reached predefined clinical stability. (pubmed.ncbi.nlm.nih.gov) In the 2021 CAP-IT trial, 814 children received study medication after discharge from emergency departments or hospital wards in the United Kingdom and Ireland. Respiratory-infection antibiotic retreatment within 28 days occurred in 12.5% of both the three-day and seven-day amoxicillin groups. The trial used an eight-percentage-point noninferiority margin, and its authors emphasized disease severity, preceding treatment, and care setting when interpreting the result. Validation therefore concerns not simply the number of days, but which patients can stop safely, how treatment failure is defined, and whether findings transfer to settings with different resistance patterns and follow-up capacity. (jamanetwork.com)\n\nArtificial intelligence research faces a related problem of transferability. Deep-learning systems can classify chest radiographs, but performance within a familiar dataset does not establish reliability in another hospital. A 2024 BMJ methodological study demonstrated limited generalizability of pediatric pneumonia classifiers trained on Guangzhou images when applied to an external dataset. Differences in the amount of abdomen included in the images contributed to erroneous classification. The study illustrates why external validation must test acquisition differences as well as patient differences. For low-resource deployment, an important unresolved question is whether algorithm-assisted interpretation improves clinical decisions under local imaging conditions, rather than merely reproducing labels assigned to a curated image collection. (bmj.com)\n\nCorticosteroid research seeks to identify which severe-pneumonia populations benefit from suppressing inflammation. In the 2023 CAPE COD trial, hydrocortisone reduced 28-day mortality from 11.9% to 6.2% among 795 intensive-care patients. (doi.org) By contrast, the 2025 REMAP-CAP corticosteroid trial stopped its fixed seven-day hydrocortisone arm for futility; observed 90-day mortality was 15.0% with hydrocortisone and 9.8% with control, with uncertainty insufficient to establish harm or exclude smaller benefits. Differences in patient selection, shock and influenza status, treatment schedules, and outcome timing leave the optimal population and regimen unresolved. (pmc.ncbi.nlm.nih.gov)\n\nReducing deaths in poorer countries also requires implementation research within health systems. A 2019 stepped-wedge trial in 12 Nigerian hospitals compared pulse oximetry introduction with a subsequent multifaceted oxygen-system intervention. The full system did not further reduce childhood mortality relative to pulse oximetry alone; secondary comparisons with baseline associated pulse oximetry introduction with lower mortality among children with acute lower respiratory infection. Economic disruption and changes in hospital activity complicated interpretation. Such studies examine the combined effects of equipment, staff training, maintenance, and clinical practice, rather than assuming that supplying oxygen equipment alone produces a predictable mortality reduction. (pubmed.ncbi.nlm.nih.gov)"
    }
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  "sources": [
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    },
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    },
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    },
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    },
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    },
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    },
    {
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    },
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    },
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      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC7107838/"
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    },
    {
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      "url": "https://pubmed.ncbi.nlm.nih.gov/22219349/"
    },
    {
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      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC9096550/"
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      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC7107997/"
    },
    {
      "title": "ACR Appropriateness Criteria: Acute Respiratory Illness in Immunocompetent Patients",
      "url": "https://acsearch.acr.org/list/TopicNarrativePdf?topicId=104"
    },
    {
      "title": "Rationale and impact — Pneumonia: diagnosis and management",
      "url": "https://www.nice.org.uk/guidance/NG250/chapter/rationale-and-impact"
    },
    {
      "title": "Guidelines for diagnosis and management of community- and hospital-acquired pneumonia in adults: Joint ICS/NCCP(I) recommendations",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC3458782/"
    },
    {
      "title": "Community-Acquired Pneumonia Clinical Pathway",
      "url": "https://www.idsociety.org/globalassets/idsa/practice-guidelines/community-acquired-pneumonia-in-adults/cap-clinical-pathway-final-online.pdf"
    },
    {
      "title": "Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study",
      "url": "https://pubmed.ncbi.nlm.nih.gov/12728155/"
    },
    {
      "title": "Pneumonia: diagnosis and management",
      "url": "https://www.nice.org.uk/guidance/ng250/resources/pneumonia-diagnosis-and-management-pdf-66144010347205"
    },
    {
      "title": "Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC6812437/"
    },
    {
      "title": "Exploratory meeting to review new evidence for Integrated Management of Childhood Illness (‎IMCI)‎ danger signs",
      "url": "https://www.who.int/publications/i/item/WHO-MCA-19.02"
    },
    {
      "title": "Oxygen therapy for children",
      "url": "https://www.who.int/publications/i/item/9789241549554"
    },
    {
      "title": "Discontinuing β-lactam treatment after 3 days for patients with community-acquired pneumonia in non-critical care wards (PTC): a double-blind, randomised, placebo-controlled, non-inferiority trial",
      "url": "https://pubmed.ncbi.nlm.nih.gov/33773631/"
    },
    {
      "title": "COVID-19 Treatment Clinical Care for Outpatients",
      "url": "https://www.cdc.gov/covid/hcp/clinical-care/outpatient-treatment.html"
    },
    {
      "title": "IDSA Guidelines on the Treatment and Management of Patients with COVID-19",
      "url": "https://www.idsociety.org/practice-guideline/covid-19-guideline-treatment-and-management?trk=public_post_comment-text"
    },
    {
      "title": "ERS/ESICM/ESCMID/ALAT guidelines for the management of severe community-acquired pneumonia",
      "url": "https://elibrary.escmid.org/download/16929464221781_s00134-023-07033-8.pdf"
    },
    {
      "title": "Hydrocortisone in Severe Community-Acquired Pneumonia",
      "url": "https://pubmed.ncbi.nlm.nih.gov/36942789/"
    },
    {
      "title": "Effect of hydrocortisone on mortality in patients with severe community-acquired pneumonia: The REMAP-CAP Corticosteroid Domain Randomized Clinical Trial",
      "url": "https://pubmed.ncbi.nlm.nih.gov/40261382/"
    },
    {
      "title": "Surviving sepsis campaign: international guidelines for management of sepsis and septic shock 2021",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/8486643/"
    },
    {
      "title": "Surviving Sepsis Campaign Guidelines 2021",
      "url": "https://sccm.org/clinical-resources/guidelines/guidelines/surviving-sepsis-guidelines-2021"
    },
    {
      "title": "Pneumonia - Treatment",
      "url": "https://www.nhlbi.nih.gov/health/pneumonia/treatment"
    },
    {
      "title": "Pneumonia - Recovery",
      "url": "https://www.nhlbi.nih.gov/health/pneumonia/recovery?exitCode=pfa"
    },
    {
      "title": "PNEUMONIA: Examination",
      "url": "https://www.paho.org/sites/default/files/who_2022_phc_children_adolescents_1.pdf"
    },
    {
      "title": "2.5.4. Children with severe acute pneumonia",
      "url": "https://tbksp.who.int/en/node/2135"
    },
    {
      "title": "Evidence and recommendations — Guideline on management of pneumonia and diarrhoea in children up to 10 years of age",
      "url": "https://www.ncbi.nlm.nih.gov/books/NBK611550/"
    },
    {
      "title": "Improved oxygen systems for childhood pneumonia: a multihospital effectiveness study in Papua New Guinea",
      "url": "https://pubmed.ncbi.nlm.nih.gov/18708248/"
    },
    {
      "title": "Placebo vs Amoxicillin for Nonsevere Fast-Breathing Pneumonia in Malawian Children Aged 2 to 59 Months: A Double-blind, Randomized Clinical Noninferiority Trial",
      "url": "https://pubmed.ncbi.nlm.nih.gov/30419120/"
    },
    {
      "title": "Infectious Diseases in Older Adults of Long-term Care Facilities: Update on Approach to Diagnosis and Management",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC5909836/"
    },
    {
      "title": "Predictors of aspiration pneumonia: how important is dysphagia?",
      "url": "https://pubmed.ncbi.nlm.nih.gov/9513300/"
    },
    {
      "title": "Frailty and hospitalization-associated disability after pneumonia: A prospective cohort study",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC7864132/"
    },
    {
      "title": "Viral Respiratory Pathogens Toolkit for Nursing Homes",
      "url": "https://www.cdc.gov/long-term-care-facilities/hcp/respiratory-virus-toolkit/"
    },
    {
      "title": "Chapter 1: Principles of Vaccination",
      "url": "https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-1-principles-of-vaccination.html"
    },
    {
      "title": "Timing and Spacing of Immunobiologics",
      "url": "https://www.cdc.gov/vaccines/hcp/imz-best-practices/timing-spacing-immunobiologics.html"
    },
    {
      "title": "Polysaccharide conjugate vaccine against pneumococcal pneumonia in adults",
      "url": "https://pubmed.ncbi.nlm.nih.gov/25785969/"
    },
    {
      "title": "Recommended Vaccines for Children",
      "url": "https://www.cdc.gov/pneumococcal/vaccines/children.html"
    },
    {
      "title": "Recommended Vaccines for Adults",
      "url": "https://www.cdc.gov/pneumococcal/vaccines/adults.html"
    },
    {
      "title": "Summary of Risk-based Pneumococcal Vaccination Recommendations",
      "url": "https://www.cdc.gov/pneumococcal/hcp/vaccine-recommendations/risk-indications.html"
    },
    {
      "title": "Hib Vaccine VIS",
      "url": "https://cdc.gov/vaccines/hcp/current-vis/hib.html"
    },
    {
      "title": "Chapter 8: Haemophilus influenzae",
      "url": "https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-8-haemophilus-influenzae.html"
    },
    {
      "title": "Who Needs a Flu Vaccine",
      "url": "https://www.cdc.gov/flu/vaccines/vaccinations.html"
    },
    {
      "title": "Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2025–26 Influenza Season",
      "url": "https://www.cdc.gov/mmwr/volumes/74/wr/mm7432a2.htm"
    },
    {
      "title": "COVID-19 vaccines",
      "url": "https://www.who.int/news-room/questions-and-answers/item/coronavirus-disease-%28covid-19%29-vaccines?adgroupsurvey=%7Badgroupsurvey%7D"
    },
    {
      "title": "Global burden of lower respiratory infections and aetiologies, 1990–2023: a systematic analysis for the Global Burden of Disease Study 2023",
      "url": "https://discovery.ucl.ac.uk/10220721/1/1-s2.0-S1473309925006899-main.pdf"
    },
    {
      "title": "Global, regional, and national incidence and mortality burden of non-COVID-19 lower respiratory infections and aetiologies, 1990–2021: a systematic analysis from the Global Burden of Disease Study 2021",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC11339187/"
    },
    {
      "title": "Global burden of lower respiratory infections during the last three decades",
      "url": "https://www.frontiersin.org/journals/public-health/articles/10.3389/fpubh.2022.1028525/full"
    },
    {
      "title": "Age–sex differences in the global burden of lower respiratory infections and risk factors, 1990–2019: results from the Global Burden of Disease Study 2019",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC9605880/"
    },
    {
      "title": "Antimicrobial Resistance with Streptococcus pneumoniae in the United States, 1997–98",
      "url": "https://wwwnc.cdc.gov/eid/article/5/6/99-0603_article"
    },
    {
      "title": "Chapter 11: Pneumococcal",
      "url": "https://www.cdc.gov/surv-manual/php/table-of-contents/chapter-11-pneumococcal.html"
    },
    {
      "title": "IDSA 2026 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections",
      "url": "https://www.idsociety.org/practice-guideline/amr-guidance?n=%40"
    },
    {
      "title": "SHEA/IDSA Clinical Practice Guidelines for Implementing an Antibiotic Stewardship Program",
      "url": "https://www.idsociety.org/practice-guideline/implementing-an-ASP/"
    },
    {
      "title": "Antibiotic-resistant Streptococcus pneumoniae",
      "url": "https://www.cdc.gov/pneumococcal/php/drug-resistance/index.html"
    },
    {
      "title": "Invasive Serotype 35B Pneumococci Including an Expanding Serotype Switch Lineage, United States, 2015–2016",
      "url": "https://wwwnc.cdc.gov/eid/article/23/6/17-0071_article"
    },
    {
      "title": "Pneumonia",
      "url": "https://library.leeds.ac.uk/special-collections/collection/2149"
    },
    {
      "title": "The Origins of the History and Physical Examination",
      "url": "https://www.ncbi.nlm.nih.gov/books/NBK458/"
    },
    {
      "title": "Hospital Centennial: Lobar Pneumonia",
      "url": "https://centennial.rucares.org/index.php?page=Lobar_Pneumonia"
    },
    {
      "title": "The Chemotherapy of Pneumonia",
      "url": "https://www.mdedge.com/content/chemotherapy-pneumonia"
    },
    {
      "title": "Alexander Fleming Discovery and Development of Penicillin",
      "url": "https://www.acs.org/education/whatischemistry/landmarks/flemingpenicillin.html"
    },
    {
      "title": "Twenty-Year Public Health Impact of 7- and 13-Valent Pneumococcal Conjugate Vaccines in US Children",
      "url": "https://wwwnc.cdc.gov/eid/article/27/6/20-4238_article"
    },
    {
      "title": "Direct and Indirect Effects of Routine Vaccination of Children with 7-Valent Pneumococcal Conjugate Vaccine on Incidence of Invasive Pneumococcal Disease — United States, 1998–2003",
      "url": "https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5436a1.htm"
    },
    {
      "title": "Progress Toward Elimination of Haemophilus influenzae Type b Disease Among Infants and Children — United States, 1987–1995",
      "url": "https://www.cdc.gov/mmwr/preview/mmwrhtml/00044142.htm"
    },
    {
      "title": "Molecular point-of-care testing for lower respiratory tract pathogens improves safe antibiotic de-escalation in patients with pneumonia in the ICU: Results of a randomised controlled trial",
      "url": "https://pubmed.ncbi.nlm.nih.gov/36096312/"
    },
    {
      "title": "A rapid host–protein test for differentiating bacterial from viral infection: Apollo diagnostic accuracy study",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC11077430/"
    },
    {
      "title": "Effect of hydrocortisone on mortality in patients with severe community-acquired pneumonia: The REMAP-CAP Corticosteroid Domain Randomized Clinical Trial",
      "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC12055926/"
    },
    {
      "title": "Oxygen systems to improve clinical care and outcomes for children and neonates: A stepped-wedge cluster-randomised trial in Nigeria",
      "url": "https://pubmed.ncbi.nlm.nih.gov/31710601/"
    }
  ],
  "license": "https://creativecommons.org/licenses/by/4.0/"
}