An infectious disease is a disease caused by an infectious agent, or pathogen, becoming established in a host and producing tissue damage or functional disturbance. Agents include bacteria, viruses, fungi, and parasites. Infection and infectious disease are not identical: an infected individual may have no symptoms, whereas disease involves clinically apparent illness. Infectious diseases are also not necessarily contagious between people; tetanus, for example, is acquired when environmental bacterial spores enter the body, usually through a wound. (ncbi.nlm.nih.gov)
Infectious agents
Bacteria are cellular organisms, some of which cause disease through tissue invasion or toxin production. Viruses depend on the machinery of a host cell for replication. Fungi include yeasts and molds capable of causing superficial or invasive infections. Parasites include protozoa, such as the organisms responsible for malaria, and helminths, or parasitic worms. These groups differ in their biological organization, life cycles, and susceptibility to medicines. (ncbi.nlm.nih.gov)
A less common category consists of prions, infectious particles composed of abnormal protein that cause certain neurodegenerative diseases. Not all microorganisms are pathogens: many inhabit the skin, mouth, and intestine without causing disease. Some organisms become harmful primarily when host defenses are weakened or when they reach an ordinarily protected body site. The presence of a microorganism therefore does not, by itself, establish infectious disease. (ncbi.nlm.nih.gov)
Transmission and host susceptibility
Transmission can be described as a chain connecting an agent’s reservoir, its route of exit, a mode of transmission, a route of entry, and a susceptible host. Reservoirs include people, other animals, and environmental habitats. Routes include direct contact, respiratory exposure, contaminated food or water, and vectors such as mosquitoes. The reservoir in which an agent normally persists may differ from the immediate source of exposure. (archive.cdc.gov)
An individual’s susceptibility reflects both exposure and biological defenses. Skin, mucous membranes, gastric acidity, and respiratory clearance impede infection. The immune system provides additional protection; immunodeficiency, malnutrition, and some medical treatments can impair it. In healthcare settings, sources of infection may include symptomatic patients, asymptomatically infected people, and individuals carrying organisms without active disease. Consequently, the absence of symptoms does not always mean an absence of transmission risk. (archive.cdc.gov)
Disease mechanisms and clinical course
Pathogens cause injury in several ways: they can destroy infected cells, invade tissues, release toxins, or provoke damaging host responses. Inflammation recruits defensive cells and molecules to an affected site, but can also contribute to tissue injury. The outcome depends on the agent’s properties, the site of infection, and the host’s response. The same organism can cause different clinical conditions when it infects different tissues. (ncbi.nlm.nih.gov)
Innate immunity supplies early defenses, while adaptive immunity includes responses directed against particular agents, involving antibodies and T cells. Some infections are cleared; others persist or become latent and may subsequently reactivate. The incubation period is the interval between exposure and symptom onset. It varies among diseases and among individuals with the same infection. Some agents can be transmitted during this presymptomatic interval. (ncbi.nlm.nih.gov)
Diagnosis
Diagnosis distinguishes the clinical illness from its microbial cause. Laboratory methods include microscopy, culture, antigen detection, antibody testing, and nucleic-acid amplification. Polymerase chain reaction and related methods detect selected microbial genetic sequences. Culture can recover organisms and permit testing of their susceptibility to antimicrobial medicines, although some agents are difficult or impossible to grow using routine methods. Different tests therefore answer different questions and have different limitations. (merckmanuals.com)
Detection of microbial material must be interpreted in its clinical context. Colonization, asymptomatic infection, and active disease are distinct possibilities. Likewise, an antibody result provides indirect evidence of the host’s response rather than directly demonstrating a living organism. Test selection and interpretation depend on the suspected pathogen and the capabilities of the method; no single routine assay detects every infectious agent. (cdc.gov)
Treatment and resistance
Antimicrobial medicines comprise antibacterial, antiviral, antifungal, and antiparasitic agents. Their activity is specific to particular organisms or biological processes. Antibiotics treat certain bacterial infections but do not treat viral infections such as influenza. Identifying the causative agent and, where applicable, its drug susceptibility supports targeted treatment rather than treating all infectious illnesses as equivalent. (who.int)
Antibiotic resistance occurs when bacteria no longer respond to antibiotics that would otherwise inhibit or kill them. The broader category of antimicrobial resistance also includes resistance in viruses, fungi, and parasites. Resistance arises through changes in microorganisms, not because the patient’s body becomes resistant. Misuse and overuse of antimicrobial medicines accelerate its development and spread, reducing available treatment options. (who.int)
Prevention and population patterns
Public health measures interrupt transmission through sanitation, infection control, and interventions directed at reservoirs or exposure routes. A vaccine prepares immune responses against a particular agent or its products, establishing immunological memory without requiring the corresponding natural disease. Vaccine effects and levels of protection vary; vaccination does not provide identical protection against every infection. (archive.cdc.gov)
Epidemiology distinguishes endemic occurrence—the usual presence of disease in a population—from an epidemic, an increase above the expected level. A pandemic is an epidemic extending across countries or continents. These describe distribution rather than individual severity. Coordinated vaccination and surveillance can profoundly alter such patterns: smallpox was declared eradicated by the World Health Organization in 1980, following international vaccination, case-finding, and containment efforts. (archive.cdc.gov)