Conjunctivitis

Conjunctivitis is inflammation of the conjunctiva, the thin cell-lined mucous membrane that covers the white of the eye and the inner surface of the eyelids, and it is commonly called pink eye. It is among the most frequent causes of a red eye. Its causes fall into two broad groups: infectious agents, chiefly viruses such as adenovirus, bacteria, and chlamydial or gonococcal organisms, and non-infectious triggers, including allergens, chemical irritants, foreign bodies, and immune-mediated disease. Because one red eye can have many different origins, clinical management depends on identifying the cause.

The condition matters for several reasons. It accounts for a large share of primary-care and eye-clinic visits, it spreads readily through households, schools, and hospitals, and it prompts a great deal of antibiotic prescribing even though most cases are viral or allergic. In a minority of cases it threatens sight: infections can damage the cornea, and neonatal and sexually transmitted forms can cause serious complications. These features link the topic to public health, antibiotic resistance, and epidemiology.

This entry first describes the anatomy of the conjunctiva and the clinical signs that help tell causes apart. It then examines each major cause group in turn, followed by diagnosis, treatment by cause, complications, transmission and prevention, epidemiology and global burden (including the related disease trachoma), historical development, and unresolved questions.

Anatomy of the conjunctiva and the ocular surface

The conjunctiva is a thin mucous membrane that lines the inner surface of the eyelids and covers the front of the eyeball, and it is one continuous sheet. It is commonly described in three regions. The palpebral conjunctiva covers the posterior surface of the eyelids and is firmly attached to the tarsal plates, so it moves little. The bulbar conjunctiva lies over the sclera and extends to the corneoscleral limbus, where it meets the clear cornea, which it does not cover. Between the two lies the fornix, a loose and flexible fold that allows the eyelids and globe to move independently. Histologically, the epithelium is non-keratinized stratified squamous to columnar tissue, and it contains goblet cells, melanocytes, dendritic cells and lymphocytes beneath a thin substantia propria. The bulbar conjunctiva is supplied mainly by branches of the ophthalmic artery, while the palpebral part draws on the external carotid system, and the two vascular networks communicate. This dense, superficial vessel network is one reason that inflammation of the conjunctiva produces the diffuse redness that clinicians describe as conjunctival injection.

Goblet cells are unicellular glandular cells scattered among the epithelial cells. They fill with mucin granules, swell as they move toward the surface, and then discharge their contents before being shed. They are most numerous in the inferonasal bulbar conjunctiva and in the tarsal conjunctiva. Their secretion is influenced by parasympathetic activity. Mucin is essential to the tear film, the thin layered fluid that coats the cornea and conjunctiva. The film has an aqueous component produced by the lacrimal glands and a mucous component from goblet cells, and mucin reduces the surface tension of the film so that it spreads evenly. Without a stable mucous layer, the epithelium dries and the surface becomes irregular, which is why reduced goblet cell density is linked to dry eye disease.

The conjunctiva also functions as an immune organ. Its lymphoid tissue, known as conjunctiva-associated lymphoid tissue (CALT), is a part of the mucosa-associated lymphoid tissue system. In the human eye, organized follicles are more common in some regions than others, and intraepithelial lymphocytes and Langerhans cells are spread throughout the surface. Langerhans cells act as antigen-presenting cells that capture material from the tear film and carry it to nearby lymph nodes. Their density is highest in the tarsal conjunctiva and declines with age. Because the tear film constantly delivers particles and microorganisms to this layer, the immune surveillance is continuous, which helps account for how quickly the surface responds to pathogens and allergens. Knop and Knop's 2000 study of human tissue concluded that the conjunctiva and lacrimal drainage system together contain all the components needed for a complete immune response, including high endothelial venules that allow lymphocytes to enter tissue.

Tears leave the surface through the lacrimal puncta, pass along the canaliculi into the lacrimal sac, and then drain down the nasolacrimal duct into the nasal cavity. This pathway matters clinically for two reasons. First, the lacrimal drainage system contains its own lymphoid tissue, termed lacrimal drainage-associated lymphoid tissue (LDALT), which shares the mucosal immune system with the conjunctiva, the lacrimal gland and the nasal mucosa. Tear flow may carry antigens from the ocular surface into this system, and lymphocytes recirculate among these sites. Second, the continuity between the eye and the nose provides a route by which agents that infect the conjunctiva can reach the upper respiratory tract, and it helps explain why some conjunctival infections occur together with nasal or pharyngeal symptoms, as seen in certain adenoviral illnesses. Because the nasolacrimal duct and the nasal mucosa are contiguous, nasal disease can also affect tear drainage and produce watering of the eye.

Clinical signs, symptoms and bedside differentiation

Redness, burning, watering and discharge occur across most forms of inflammatory conjunctival disease, so these features rarely establish a diagnosis on their own. Common clinical findings include itching, burning, and ocular discharge; the clinical presentation sometimes suggests the underlying etiology. The typical pattern is diffuse injection that extends around the eyeball and under the upper and lower lids, and the eyelids may adhere to each other on waking.

The character of the discharge is usually the most useful bedside clue. Viral disease tends to produce thin, watery or mucoserous discharge rather than purulent discharge. By contrast, bacterial pink eye produces thick, yellow or green pus that often crusts the eyelids shut overnight. A mucoid, stringy or ropy discharge is highly characteristic of allergy or dry eyes. Copious purulent discharge should prompt consideration of Neisseria gonorrhoeae infection. Discharge pattern is a tendency rather than a rule, and mixed presentations are common in practice.

Itching is the symptom that most strongly points toward allergy. Without itching, the diagnosis of allergic conjunctivitis becomes doubtful. Allergic disease also tends to involve both eyes at once, whereas viral conjunctivitis often begins in one eye and spreads to the other within days. Chemosis, the swelling of the conjunctiva, and eyelid edema often accompany allergic disease. Classic signs of allergic conjunctivitis include injection of the conjunctival vessels as well as varying degrees of chemosis and eyelid edema. Allergies are the most common cause of conjunctival chemosis. Chemosis is nonetheless not specific to allergy, and it can also be seen in infections, particularly adenovirus-related viral conjunctivitis, which may produce watery discharge and glassy, swollen eyes.

Preauricular lymph node enlargement leans toward a viral or chlamydial cause. Viral or chlamydial inclusion conjunctivitis typically presents with a small, tender, preauricular or submandibular lymph node. Palpable adenopathy is rare in acute bacterial conjunctivitis, the exception being hyperacute conjunctivitis caused by Neisseria species. Morning eyelid adhesion, meanwhile, is typically associated with bacterial infection, since patients with acute bacterial conjunctivitis often complain of eyes that are matted and stuck shut in the morning.

Pain, photophobia, reduced vision and corneal changes are the features that shift the assessment away from simple conjunctivitis. Conjunctivitis typically presents without pain unless the cornea is involved, so significant pain implies a deeper or corneal process. Clinicians are advised that pain, loss of vision or photophobia require immediate referral to an ophthalmologist. Other red flags include a unilateral red eye, changes in visual acuity, and a ciliary flush, a pattern of injection in which the redness is most pronounced in a ring at the limbus. These signs point toward keratitis (inflammation of the cornea), iritis, or angle closure glaucoma. Angle-closure glaucoma can also present with headaches, nausea, vomiting, ocular pain, decreased vision, light sensitivity, and seeing haloes around lights. Any of these features calls for a full examination rather than a presumptive diagnosis of pink eye.

Viral conjunctivitis

Adenoviruses cause most viral conjunctivitis. Estimates in the literature range from about 65% to 90% of cases, and the spread largely reflects differences in study populations and laboratory methods. The two classic adenoviral syndromes are epidemic keratoconjunctivitis (EKC) and pharyngoconjunctival fever (PCF). A third form, acute hemorrhagic conjunctivitis (AHC), is caused mainly by picornaviruses rather than adenoviruses. Molecular studies also show that the relative frequency of each agent varies between settings, and that a meaningful minority of cases, roughly one in four in one prospective study, are not adenoviral.

EKC is the more severe of the adenoviral forms. It is most often linked to adenovirus serotypes 8, 19 and 37, and it can involve the cornea, with epithelial lesions and subepithelial infiltrates. These infiltrates are thought to reflect a delayed hypersensitivity response to viral antigens, and in a substantial share of patients they persist for weeks to months and can temporarily reduce visual acuity. PCF is associated mainly with serotypes 3, 4 and 7. It combines conjunctivitis with fever, pharyngitis and tender enlargement of the preauricular lymph nodes, and this node enlargement is a useful clue toward adenoviral infection. PCF spreads readily among people in close contact, such as those in households, schools and workplaces.

AHC is an epidemic, highly contagious illness marked by sudden pain, swelling and redness of the eyes, often with subconjunctival hemorrhage. It is mostly caused by enterovirus 70 and coxsackievirus A24, and large epidemics have been recorded in tropical and subtropical regions. Its clinical picture resembles adenoviral disease but is generally more hemorrhagic and more severe.

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) cause less common but more consequential forms. HSV keratoconjunctivitis may affect only one eye, is associated with blister-like skin lesions in some patients, and can produce characteristic corneal dendrites on fluorescein staining. Primary HSV and primary VZV infections mainly affect young children, whereas herpes zoster ophthalmicus results from reactivation of latent VZV and can occur at any age. Several systemic viruses also involve the conjunctiva. Measles, rubella, mumps, chickenpox and Epstein–Barr virus can produce conjunctivitis as part of a wider illness, and COVID-19 and some other respiratory viruses have been recognized as additional causes.

Most viral conjunctivitis is self-limiting. Uncomplicated adenoviral disease usually resolves within two to three weeks, although corneal involvement in EKC may peak later, around the third or fourth week. Adenoviruses are highly contagious, spread through direct contact and contaminated objects, and are unusually resistant to common disinfectants and physical agents. Because the clinical features of different viral agents overlap considerably, the cause is often presumed rather than confirmed, and laboratory testing is generally reserved for epidemics, severe or recurrent cases, and patients who do not respond to supportive care.

Bacterial conjunctivitis

In acute disease, clinicians most often isolate three organisms: Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae. Age shifts the distribution. Infections with S. pneumoniae and H. influenzae are more common in children, whereas S. aureus most frequently affects adults. Adult case series identify S. aureus first, followed by pneumococci and Haemophilus species. In children with mild disease, S. pneumoniae and Moraxella species are the leading agents, and H. influenzae was a prominent causative agent before the availability of Hib vaccine. A pediatric study of 67 children with bacterial conjunctivitis and concurrent ear or sinus infection found H. influenzae in about 70% of isolates, with Moraxella and S. aureus far behind.

Gram-negative organisms form a smaller group but carry greater risk. Acute disease has been linked to Pseudomonas aeruginosa, Moraxella lacunata, Streptococcus viridans, and Proteus mirabilis. Pseudomonas and Serratia marcescens are associated with more virulent courses in which the cornea can be invaded quickly, which is why these infections are treated as higher-risk than routine cases. Even among common organisms, the course is not uniform. Most cases are self-limited within 1-2 weeks of presentation, but in cases caused by highly virulent bacteria (eg, S. pneumoniae, N. gonorrhoeae, and H. influenzae) the disease can behave differently.

The typical presentation is a sticky, mucopurulent illness. Patients usually report tearing, irritation, and discharge, without preauricular lymphadenopathy as an early feature. The discharge is thick and often crusts the lashes, and the lids may be stuck together by a mucopurulent exudate on awakening. The conjunctiva typically appears pink (not red). Bilateral matting and lid adherence without itching are strong positive predictors of bacterial conjunctivitis. Severity varies by organism: severe bacterial conjunctivitis, characterized by pronounced conjunctival infection and copious purulent discharge, usually is caused by Neisseria gonorrhoeae, Neisseria meningitidis, S. aureus, S. pneumoniae, and, in children younger than 5 years, H. influenzae.

Culture is frequently unhelpful. Diagnosis is usually made on clinical grounds, and Gram stain and cultures on chocolate and blood agar are primarily used in cases of atypical conjunctivitis, such as hyperacute or chronic/nonresponding, as well as in neonatal conjunctivitis. Negative results are common, largely because some causative bacteria, including fastidious Haemophilus species, grow poorly on routine laboratory media, and prior antibiotic exposure can further reduce yield. A negative culture therefore does not exclude a bacterial cause, and the lack of an isolate makes it harder to tailor antibiotic therapy to a specific organism. Clinicians commonly start with empirical broad-spectrum treatment instead. When there is no response, a poor clinical response after 2 or 3 days indicates that the cause is resistant bacteria, a virus, or an allergy. Culture and sensitivity studies should then be done. Where the distinction between viral and bacterial disease remains unclear, viral culture or polymerase chain reaction (PCR) can help differentiate viral versus bacterial conjunctivitis.

Chlamydial and gonococcal conjunctivitis

Adult inclusion conjunctivitis is the ocular form of infection with Chlamydia trachomatis. The disease is usually sexually acquired, and in rare cases it can be acquired by swimming in a contaminated swimming pool. Clinically it tends to be chronic and one-sided, with mucopurulent discharge. Symptoms lasting more than three weeks, a marked follicular response on the tarsal conjunctiva, and failure to respond to topical antibiotics help distinguish it from other bacterial conjunctivitis. Confirmation relies on immunofluorescent staining, nucleic acid amplification tests (NAAT), and special cultures, with PCR-based methods being the most common in practice.

Because the infection is linked to genital tract disease, systemic rather than topical therapy is the usual approach. A single 1 g oral dose of azithromycin is now the standard treatment in adults, and alternatives include doxycycline 100 mg twice daily for seven days, or erythromycin 500 mg four times a day for seven days. Tetracyclines carry restrictions: they should be avoided in children younger than age 8 and in pregnant or lactating women. Sex partners also require treatment.

Gonococcal conjunctivitis, caused by Neisseria gonorrhoeae, is a far more fulminant process. Its discharge is so profuse that some clinicians have likened it to a fountain of pus, and there is a documented increase in infections during spring and summer. Patients typically present with acute pain, redness, and hyperacute profuse purulent discharge, and examination may show conjunctival chemosis, globe tenderness, eyelid edema, and preauricular lymphadenopathy. The organism's pili help it attach to the epithelium, and it can then adhere to and penetrate intact corneal epithelium. Corneal perforation and vision loss can occur within 24 hours. In adults the eye is usually infected by autoinoculation, often via contaminated hands after contact with infected genital secretions.

Gram staining of purulent discharge may show gram-negative diplococci, but culture is the definitive test. Because of the aggressive course, treatment must be started before culture results are available. The standard regimen is ceftriaxone 1 g intramuscularly in a single dose, as recommended by the CDC. Ceftriaxone does not treat chlamydia, so patients with chlamydia need a second antibiotic, which is azithromycin, 1 g orally. Resistance remains a concern: strains resistant to quinolones have emerged, although cephalosporin resistance in the United States is rare. Corneal involvement changes the management. One clinician estimated that about 40% of the time, there is an ulceration or a perforation, and such patients require intravenous ceftriaxone because the intramuscular route doesn't work quick enough.

Syphilis and other sexually transmitted infections can also cause ocular inflammation, so a sexual history forms part of the diagnostic workup. Treatment for one such infection does not cover the others, and screening is therefore usually broader than the eye findings alone would suggest.

Neonatal conjunctivitis

Eye infection in the first four weeks of life is most often classed as ophthalmia neonatorum, and it typically reflects bacterial exposure during birth. Ophthalmia neonatorum, or neonatal conjunctivitis (NC), is the most common disease of the eye for newborns during the first 4 weeks of life and typically results from exposure to bacteria during birth. Infants usually meet these organisms as they pass through the birth canal, which is why maternal infection matters so much. Chlamydial ophthalmia, caused by Chlamydia trachomatis, is the most prevalent bacterial cause, accounting for up to 40% of conjunctivitis cases in neonates younger than 4 weeks. The maternal carriage rate is itself variable, with one estimate placing it between 2% to 20%. Chlamydia trachomatis is therefore the central organism to consider in this age group.

Gonococcal disease is much rarer but carries the greatest threat to sight. Neisseria is the most visually threatening cause, yet gonococcal ophthalmia (conjunctivitis caused by Neisseria gonorrhoeae) accounts for less than 1% of cases. The Neisseria gonorrhoeae infection is the one that has historically driven prophylaxis policy. Other organisms also appear regularly. Other bacteria, including Streptococcus pneumoniae, Staphylococcus aureus, and nontypeable Haemophilus influenzae, account for many of the remaining bacterial cases, and since the introduction of universal gonococcal and chlamydia prophylaxis, other etiologies for neonatal conjunctivitis such as Escherichia coli have become more common. Chemical irritation from prophylactic drops and herpetic infection are also listed among the usual causes.

Clinical features and timing help separate the main infections, although laboratory testing is needed to confirm the organism. Gonococcal disease declares itself early: it causes an acute purulent conjunctivitis that appears 2 to 5 days after birth or earlier with prelabor rupture of membranes. The eyelids are typically swollen and the discharge can be heavy, and neonatal infections are typically bilateral. Chlamydial disease generally appears later. Its onset is commonly placed 5 days after birth to 2 weeks, reflecting its longer incubation period. Because the first month defines the condition, the onset window narrows the differential rather than settling it. Diagnosis is clinical and usually confirmed by laboratory testing. Early culture has been proposed as a way to start targeted treatment sooner, since rapid identification can help preserve vision.

Prophylaxis has transformed the outlook for newborns. Before the 1880s, gonococcal infection was a major source of permanent blindness in infants, and in 1881 Carl Credé introduced a single drop of silver nitrate into each newborn's eyes shortly after birth. Reported results vary between studies. One account gives a fall from 8% to 0.3%, while another reports a reduction from 13.6% to 0.05%. Credé's original solution was 2%, but the concentration was later lowered to 1% because of chemical irritation. Silver nitrate itself caused problems: chemical irritants such as silver nitrate can cause chemical conjunctivitis, usually lasting 2–4 days. It was also less reliable against chlamydia, and chlamydia is now the most common cause of ophthalmia neonatorum in developed countries. Its use declined, and the agents used for ocular prophylaxis have shifted from silver nitrate to erythromycin or tetracycline ophthalmic ointments and povidone/iodine. The WHO now recommends that topical ocular prophylaxis for prevention of gonococcal and chlamydial conjunctivitis be given to all neonates.

Newborns with suspected infection need urgent assessment and referral, because delay can allow serious damage. Untreated cases may develop corneal ulceration, which may perforate, leading to corneal opacification and staphyloma formation. Treatment is directed by the organism identified, using antibiotics matched to the infection. Prophylaxis is not infallible: a 2025 report from Japan described a gonococcal infection caused by a levofloxacin-resistant Neisseria gonorrhoeae bacteria strain, co-infected with Chlamydia trachomatis bacteria. That case followed the discontinuation of an older preparation, which prompted some institutions to adopt fluoroquinolone-based ophthalmic agents for neonatal prophylaxis. The episode underscores why screening and antibiotic resistance monitoring remain part of neonatal care.

Allergic conjunctivitis

Allergic conjunctivitis is an IgE-mediated hypersensitivity reaction to environmental allergens, and it is best understood as a type I hypersensitivity response. Sensitization begins when allergen-presenting cells activate Th2 lymphocytes, which release interleukin-4 and interleukin-13. These cytokines drive B cells to switch to IgE production, and the resulting allergen-specific IgE binds to high-affinity FcεRI receptors on conjunctival mast cells and basophils. Once sensitized, the conjunctiva is primed: a later allergen contact cross-links the bound IgE, which triggers mast cell degranulation.

The response unfolds in two phases. The early phase is dominated by preformed mediators, chiefly histamine, along with tryptase, which act within minutes to produce itching, vasodilation and conjunctival hyperemia, as well as chemosis. Histamine acts on H1 receptors for itch and on H2 receptors for vascular changes. The late phase, which can start hours later and last longer, is driven by newly synthesized lipid mediators such as prostaglandins and leukotrienes, together with cytokines and chemokines. These recruit eosinophils and further Th2-associated leukocytes into the conjunctival stroma, so symptoms can persist after histamine has been cleared. This two-phase model explains why antihistamines relieve acute itching quickly but may not control prolonged inflammation on their own.

Clinical subtypes are usually grouped by duration and severity. Seasonal allergic conjunctivitis is the most common form in the general population and is typically triggered by pollen. Perennial allergic conjunctivitis produces symptoms throughout the year, usually in response to indoor allergens such as dust mites, animal dander and molds. Vernal keratoconjunctivitis and atopic keratoconjunctivitis are chronic, more severe forms with a strong type-2 inflammatory component; they are seen mainly in specialist ophthalmology centers and carry a risk of corneal complications. Giant papillary conjunctivitis, which is associated with contact lenses or ocular prostheses, is driven by mechanical irritation of the upper tarsal conjunctiva as well as by immune mechanisms. Not all chronic forms are uniformly IgE-dependent, and the role of epithelial alarmins such as TSLP, IL-33 and IL-25 is an area of active study.

Allergic conjunctivitis is strongly linked to other atopic diseases, especially allergic rhinitis, which is often described as rhinoconjunctivitis. Ocular symptoms are reported in roughly 30% to 71% of people with allergic rhinitis. Prevalence estimates in the general population range from about 6% to 30%, and a pooled figure of about 31.7% has been reported among people with atopic disease. Survey results differ considerably. A US National Health and Nutrition Examination Survey (1988–1994) suggested that about 35% of people had ocular allergy symptoms, whereas registry data from the American Academy of Ophthalmology's IRIS database found prevalence of only 2.8% to 3.5%. A 2013–2016 cross-sectional study in Tehran reported 15.9%, and a Korean adult study reported 5.6%. Incidence and prevalence also peak in spring, are higher among women, and are highest among people aged 11 to 30. Because the figures depend on case definitions, season and study design, they should be compared with caution.

Non-infectious and toxic causes

Conjunctivitis is not always caused by a microbe. Irritant conjunctivitis is a non-infectious form of the disease triggered by a brief mechanical or chemical insult, and it can be acute, relapsing or chronic. Chemical injury is the most common route. Exposure damages the surface epithelial cells, and the depth of injury depends on the strength and pH of the agent. Alkaline substances penetrate more deeply than acids, which is why alkali exposure is generally regarded as more dangerous. Household products are a frequent source, including bleach and ammonia-based cleaners, oven cleaners and drain openers, disinfectant sprays and aerosol products, and detergents and soaps. Aerosols carry a particular risk because fine droplets can reach the ocular surface easily.

Some toxic reactions come from medications and lens-care products rather than from a single splash. Benzalkonium chloride and other preservatives in eye drops can produce a toxic reaction that is caused by the use of certain eye care products or eye medications for a long period of time. Milder irritants may act slowly; symptoms may develop over days, weeks, or even months of repeated use. The reaction is clinically similar to allergy, but it is not immune-mediated, which is why it is often confused with allergic conjunctivitis.

Swimming pool irritation is a common everyday example. Chlorine reacts with sweat, urine and body oils to form chloramines, and these compounds upset the eye's tear film, leaving the surface dry and exposed. The tear film is the surface layer that protects the cornea and conjunctiva, so its disruption explains why the injury is mainly a loss of surface protection rather than a direct infection. A poorly maintained pool with pH imbalance can make the irritation worse, and the damage may also open the way for waterborne microbes.

Mechanical and environmental factors form a second group. Dry eye, whatever its cause, is associated with irritant conjunctivitis, as are chemical splashes, irrigation of the eye, abrasions and blunt trauma. A retained foreign body can keep the eye red after it has been removed; redness and discharge may persist for 12 to 24 hours after the foreign body has been spontaneously expelled. Chronic exposure to ultraviolet light and dry air is also thought to aggravate surface inflammation, although these links are less direct than those for chemical injury.

Chronic cicatrizing conjunctivitis is a separate category, in which inflammation leaves scars behind. Ocular surface burns, Stevens–Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN), atopic keratoconjunctivitis or vernal keratoconjunctivitis, and mucous membrane pemphigoid can lead to this scarring and are leading causes of limbal stem-cell deficiency. Ocular cicatricial pemphigoid, now usually grouped under mucous membrane pemphigoid, is an autoimmune disorder with the same scarring pattern. In suspected SJS, a score of greater than 5 was found to be highly sensitive and specific in identifying SJS as the underlying cause of cicatrizing disease.

Neoplasia must also be considered when inflammation does not respond to treatment. Although uncommon, ocular surface malignancies may involve the conjunctiva diffusely and present as chronic conjunctivitis. The main example is ocular surface squamous neoplasia (OSSN), a spectrum that ranges from dysplasia and carcinoma-in-situ to squamous cell carcinoma. Its two main risk factors are UV-B light and human papillomavirus. Other conditions can mimic chronic conjunctivitis, including sebaceous carcinoma and conjunctival lymphoma. A persistent unilateral lesion or one that resists standard therapy therefore warrants biopsy or cytology rather than repeated courses of drops. Any ocular surface lesion refractory to standard medical treatment should raise suspicion for a malignant process.

Diagnosis and differential diagnosis

Diagnosis of conjunctivitis is usually clinical, based on the history and a focused examination. The etiology can usually be determined by a careful history and an ocular examination, but culture is occasionally necessary. In uncomplicated disease, visual acuity is preserved. Conjunctivitis does not typically impair vision and pupils should still be reactive to light. Reduced vision therefore counts as a warning sign for a more serious condition, not as a feature of ordinary conjunctivitis.

Because "red eye" has many causes, the examination separates conjunctivitis from other conditions by checking pain, photophobia, pupil reactions and the pattern of redness. Severe pain or headaches may indicate sight-threatening acute angle glaucoma, scleritis or corneal ulcer. In contrast, common causes of red eye such as conjunctivitis normally cause mild pain, irritation and/or foreign body sensation. Pain that worsens with pupillary constriction, or unequal pupils, has been associated with serious non-conjunctival disease in prospective data. Recurrent or treatment-resistant redness calls for re-examination, since the working diagnosis may be wrong. In one reported case, bilateral symptoms persisted despite two antibiotic courses, and the absence of mucopurulent discharge led clinicians to reconsider the diagnosis, with ocular rosacea later identified as the underlying cause.

Slit-lamp examination with fluorescein staining is the central step in excluding corneal involvement. Fluorescein should be instilled in the conjunctival sac and the eye viewed with the cobalt blue light of the slit lamp, which reveals epithelial defects that would turn a routine case into a potentially sight-threatening one. This matters because keratitis changes both the urgency of referral and the choice of treatment. In the classic presentation of herpes simplex keratitis, staining shows branching dendritic lesions, while punctate erosions point toward other causes. Keratitis usually warrants specialist review within a short interval. Staining also helps identify other conditions that mimic conjunctivitis, including corneal abrasion, which fluorescein highlights as epithelial loss.

Microbiological sampling is not routine. Swabs and cultures are generally kept for severe disease, recurrent or chronic cases, neonatal infection, and cases that fail to respond to empirical treatment, where they can guide therapy. Gram staining is of limited value in ordinary primary care, because many causative organisms are not reliably identified this way, and this limitation is one reason why cultures often return negative results. Clinicians rely instead on the pattern of discharge, laterality, lymph node status and the response to treatment.

Several other conditions must be distinguished from conjunctivitis because they threaten vision. Acute angle-closure glaucoma, uveitis, scleritis, keratitis and orbital cellulitis are the principal examples. Subconjunctival hemorrhage, by contrast, appears as a painless bright red patch on the sclera and is generally benign. Dry eye, blepharitis and episcleritis also produce red eyes but have different histories and examination findings. Distinguishing these conditions is therefore central to the decision about whether a patient can be managed in primary care or needs urgent ophthalmology referral.

Management of viral and self-limiting cases

Viral infection accounts for most acute infectious conjunctivitis in adults. In most cases, it is mild and clears up without treatment. Drug therapy is therefore not a routine requirement for uncomplicated illness, and the main task is to relieve symptoms while the viral infection runs its course.

Symptom control relies on simple measures. Lubricating drops, cool compresses, and oral analgesics for significant pain are the usual options, and artificial tears, topical antihistamines, topical steroids, oral analgesics, or cold compresses may be used to mitigate symptoms. Guidance is not uniform on antihistamines. One review holds that topical vasoconstrictors and antihistamines are generally not indicated, because they are minimally helpful and may cause rebounding of symptoms, as well as local toxicity and hypersensitivity. Other guidance lists them among the supportive options, and the difference reflects the weight given to modest benefit against rebound effects.

Specific antiviral drugs have little established role in adenoviral disease. No evidence exists that demonstrates the efficacy of specific antiviral agents other than topical ganciclovir. Oral antiherpetic drugs have no demonstrated effect on adenoviral eye disease. Because adenovirus is hardy, infection control is the most practical intervention: this is a hearty virus that can survive for many weeks on a countertop or similar surface if careful disinfection doesn't occur.

Antibacterial agents have no effect on the course of viral disease. Viral conjunctivitis will not respond to antibacterial agents, and mild bacterial conjunctivitis is likely to be self-limited. Antibiotics may still be considered in a narrow group of patients, since for patients who may be susceptible, a topical astringent or antibiotic may be used to prevent bacterial superinfection. Unneeded prescribing carries its own costs, because it cause[s] unnecessary ocular toxicity and allergic reactions, and promote[s] antimicrobial resistance. This is the reason for the stewardship principle that antibiotics should be reserved for cases that need them.

The picture changes for infections caused by herpes simplex virus and varicella-zoster virus, which need specific antiviral treatment. Both HSV and HZV infections can be treated with oral antivirals, including acyclovir, valacyclovir and famciclovir. In herpes zoster ophthalmicus, nearly all patients with acute manifestation require treatment with systemic antiviral medications, as the disease process is not limited to the ocular surface. Benefit is greatest when therapy begins early, and antiviral medications such as acyclovir, valacyclovir, and famciclovir remain the mainstay of therapy and are most effective in preventing ocular involvement when begun within 72 hours after the onset of the rash.

Corticosteroid drops require particular caution in these settings. Extreme caution should be taken when using corticosteroids, as they may worsen an underlying HSV infection, so HSV should be excluded before steroids are started. Conjunctivitis in zoster is often self-resolving, since the conjunctiva appears injected and edematous, often with petechial hemorrhages. A secondary bacterial infection can complicate the course: secondary infection, usually Staphylococcus aureus, may develop and should be treated with broad-spectrum topical and/or systemic antibiotics. Specialist input is generally advisable for these cases, and patients with severe disease who have corneal epithelial ulceration or membranous conjunctivitis should be re-evaluated within 1 week.

Management of bacterial, chlamydial and gonococcal infections

Topical antibiotic drops or ointments remain the core of treatment for bacterial conjunctivitis. When the organism is not yet known, clinicians commonly start with a broad-spectrum agent that covers the frequent Gram-positive and Gram-negative pathogens, then narrow or change the drug once the clinical course or laboratory results are available. Because bacterial disease is usually self-limiting in mild cases, the choice of agent and the decision to treat at all depend on severity and on the risk of corneal involvement.

Gonococcal and chlamydial infections cannot be managed with eye drops alone, because these organisms often spread beyond the eye and frequently coexist with genitourinary infection. Systemic antibiotics are therefore required. For adults with gonococcal conjunctivitis, one reference describes a single intramuscular dose of ceftriaxone 1 g, combined with a single oral dose of azithromycin 1 g to cover possible chlamydial co-infection. Doxycycline 100 mg twice daily for seven days is the alternative when azithromycin cannot be used. Fluoroquinolones are no longer recommended for gonococcal disease because resistance has become widespread. A separate reference lists 1000 mg of ceftriaxone for gonococcal conjunctivitis, whereas the CDC's regimen for uncomplicated genital gonorrhea uses 500 mg, so ocular dosing should be checked against current local guidance. Where chlamydial infection has not been ruled out, guidance calls for doxycycline given alongside the injectable cephalosporin.

Care of these infections extends beyond the patient. Sexual partners should be evaluated and treated, and screening for other sexually transmitted infections, including syphilis, is part of the workup. Test-of-cure is generally not required for gonococcal conjunctivitis, but clinicians should monitor for treatment failure, which remains uncommon for ceftriaxone in the United States.

Contact lens wearers with bacterial conjunctivitis need additional measures. Lens use should stop until the infection resolves, topical antibiotics should be started, and an ophthalmologist should assess the eye, since bacterial organisms can invade the cornea quickly and produce keratitis.

Antimicrobial stewardship is a central principle in this setting. The CDC's 2020 update removed the routine azithromycin component from the genital gonorrhea regimen, citing rising azithromycin resistance and the possible harm of dual therapy to commensal organisms. Limiting unnecessary antibiotic prescribing, including for self-limiting viral or allergic disease, is widely advised to slow the spread of antibiotic resistance.

Management of allergic conjunctivitis

Care is usually layered. It starts with reducing contact with the allergen and with simple supportive steps. Clinical reviews describe cold compresses and tear substitutes as the basic measures, used alongside allergen avoidance (Non-specific measures to ameliorate symptoms conventionally include: cold compresses, eyewashes with tear substitutes, and avoidance of allergens.). For mild disease these steps may be enough. When they are not, pharmacological treatment usually begins with topical agents.

Topical antihistamines are the usual next step. Their target is the H1 receptor for histamine, and blocking it reduces itching and vasodilation (Ophthalmic antihistamines competitively bind to the histamine H1 receptor to reduce itching and vasodilation and inhibit mast cell degranulation). Side effects are generally mild, most often burning, stinging or eye pain (Adverse events for this class are generally mild and include burning, stinging, and/or pain in the eye.). Mast cell stabilizers work upstream. They keep mast cells from releasing mediators, but they do little for symptoms already present, so they must be used prophylactically and for longer periods (mast cell stabilizers generally do not relieve existing symptoms and are to be used on a prophylactic basis to prevent mast cell degranulation with subsequent exposure to the allergen. Therefore, they need to be used long term in conjunction with various other classes of medications.). Many current products combine both actions, and these dual-action agents are commonly regarded as first-line therapy for seasonal and perennial disease (First-line therapy for most patients suffering from SAC and PAC is topical dual-acting antihistamines/mast cell stabilizers). A systematic review found that topical antihistamines and mast cell stabilizers outperformed placebo with no serious adverse effects, but it judged the evidence insufficient to rank individual drugs (There is insufficient evidence to compare the effectiveness of specific topical medications. No serious adverse effects are associated with these medications.). For systemic use, second-generation oral antihistamines are preferred because they cause less sedation (the use of second-generation systemic antihistamines, with fewer adverse events and a much better profile in terms of sedation, is recommended).

Topical corticosteroids are added for more severe or persistent symptoms. Because of their side effects, including raised intraocular pressure, clinicians keep both the strength and the duration of steroid courses to the minimum that works (topical steroids should be prescribed at the lowest effective concentration and for the shortest duration possible.). This is why steroids are generally reserved for the more severe forms (Corticosteroids might be effective but, because of side effects, are reserved for more severe forms of allergic conjunctivitis such as vernal keratoconjunctivitis (VKC) and atopic keratoconjunctivitis (AKC).).

Chronic vernal and atopic forms are managed with more specialized care. In vernal disease, mast cell stabilizers are regarded as a mainstay of treatment and can be used long term, although most patients with significant symptoms eventually need topical steroids (Mast cell stabilizers, with antihistamine effects, are perhaps the mainstay of treatment of VKC and are safe for long-term use. However, topical corticosteroids generally become necessary for most patients with significant symptoms.). Immunomodulatory drugs are reserved for severe keratoconjunctivitis (corticosteroids and immunomodulators are reserved for severe cases of keratoconjunctivitis). Corneal involvement in vernal disease can cause severe pain and visual loss (VK is a more serious subset of VKC with significant corneal involvement, potentially leading to severe pain and vision problems.), and surgery may be considered in severe VKC or AKC (Surgical intervention may be indicated in severe cases of VKC or AKC.).

Allergen immunotherapy is considered when symptoms persist despite standard measures and sensitization to a specific allergen has been confirmed. The most studied approach for the eye is sublingual administration. Pooled data from 42 trials involving 3,958 participants, reported in a 2011 Cochrane meta-analysis, indicate that sublingual treatment lowers ocular symptom scores compared with placebo, although it did not significantly reduce the use of eye drops (Forty two trials including 3958 participants (n=2011 SLIT and n=1947 placebo) had data suitable for inclusion in the meta-analysis. SLIT induced a significant reduction on both total ocular symptom scores (SMD: −0.41; 95%CI: −0.53 to −0.28; I2: 59%) and individual ocular symptoms scores; No significant reduction was observed on eye drops use). Safety reviews of sublingual immunotherapy describe a profile largely free of serious systemic reactions, which is a key reason it is preferred over injections (Sublingual immunotherapy (SLIT) has met such need while maintaining a clinical efficacy comparable to SCIT. In fact, the safety profile, as outlined by a systematic revision of the available literature, was substantially free from serious systemic reactions.). Whether the benefit persists after treatment stops has not been firmly established, and reviewers call for larger, long-term studies (There is a need for further large rigorously designed studies that study long-term effectiveness after discontinuation of treatment and establish the cost-effectiveness of SLIT.).

Complications, prognosis and chronic outcomes

The cornea is the tissue whose involvement most affects the outcome. When infection or inflammation reaches it, the risk to sight rises sharply. Gram-negative organisms show this clearly: Pseudomonas can cause corneal ulceration and perforation quickly, even though it is rarely implicated in neonatal disease, and untreated Pseudomonas keratitis even can lead to endophthalmitis. Gonococcal infection behaves similarly. Left untreated, it can dissolve corneal tissue, perforate the globe's clear front surface, and in some cases end in blindness that requires corneal transplantation. Herpes simplex adds a different pattern, in which skin vesicles and keratitis occur together.

Severe or untreated disease can leave lasting damage. In one reported adult case of gonococcal keratitis, follow-up at one month showed that the patient maintained low visual acuity in the right eye, and biomicroscopy showed a cornea with a large cicatricial leukoma area. Scarring of this kind is the main route to permanent visual loss in corneal disease. Uveitis, chronic dry eye and reduced vision are the other sequelae the section's broader clinical picture recognizes, and they tend to follow the same pattern: the more the deeper ocular tissues are involved, the poorer the eventual outcome.

Most uncomplicated cases recover without lasting harm. Prognosis depends on the causative agent and on whether the cornea has been affected, so a mild surface infection that spares the cornea carries a very different outlook from a keratitis-associated infection. In allergic disease the outlook is also generally favorable: complications are uncommon, although symptoms may recur. Recurrence is driven largely by exposure, since recurring exposure to allergens will likely trigger the same symptoms in the future.

Long-standing allergic inflammation can cause cumulative ocular surface damage. Persistent inflammation may lead to recurrent epithelial defects, corneal scarring, shield ulcers, conjunctival fibrosis, keratoconus secondary to chronic eye rubbing, and permanent visual impairment, particularly in the severe vernal and atopic forms. Those forms are classed as severe because they can cause keratopathies, such as shield ulcer or corneal plaque, leading to visual morbidity in some cases. Their chronicity is linked to atopic disease: because atopic dermatitis does not heal completely, associated severe atopic keratoconjunctivitis and vernal keratoconjunctivitis often require long-term treatment. Treatment itself carries risk; some medications used for allergic conjunctivitis may also increase the risk of cataracts, and chronic preservative exposure may contribute to tear film instability, goblet cell loss, ocular surface inflammation, corneal epithelial toxicity, and worsening dry eye disease.

Because of these trajectories, clinicians emphasize early control. Management guidance holds that early recognition and aggressive control of inflammation can reduce chronic ocular surface remodeling and improve long-term visual outcomes. Long-term follow-up studies of topical tacrolimus in severe allergic disease have tracked patients for an average use period of 8.4 ± 2.9 years, reflecting how the chronic forms are managed over years rather than weeks.

Transmission, outbreaks and prevention

Viral conjunctivitis spreads mainly through hand-to-eye contact and through fomites, meaning objects that carry infectious material. Transmission is primarily via hand to eye contact and fomites. Bacterial disease follows a similar pattern, since fingers that have touched discharge can carry organisms to the other eye or to another person.

Adenovirus, the most frequent cause of viral conjunctivitis, has a wider set of routes. Direct contact with ocular secretions is the most common mode of transmission, although respiratory droplets have also been proposed as a source. Transmission can also occur through contact with ophthalmic care providers and their medical instruments. Eye examination equipment, eye solutions, inadequately chlorinated swimming pools, and other contaminated objects have all been implicated. Fecal-oral and fomite spread have been documented as well, and outbreaks of adenoviral keratoconjunctivitis have involved ophthalmic solutions and a roller towel used for drying faces and hands.

The period of contagiousness is closely tied to the clinical course. A person is thought to shed the virus while their eyes are red and tearing. In adenoviral disease, the illness is biphasic: an initial infective phase is followed by an inflammatory phase that typically begins about 7–10 days after the initial infection as the virus continues to shed. Overall, most cases of viral conjunctivitis are highly contagious for 10-14 days. Shedding can last much longer in immunocompromised patients, which matters in hospitals, where fomite transmission has been reported.

Outbreaks are most likely in crowded settings. Viral infections are more prevalent in densely populated, overcrowded areas, or in the presence of poor hygiene, or both. In enclosed populations, bacterial outbreaks have been documented; one review notes that such outbreaks affected 13 to 28% of the population in settings such as college campuses and daycare centers. Public health responses to epidemic forms therefore rely on outbreak reporting, case tracing, and infection control in schools, hospitals and households.

Prevention rests on basic hygiene. Frequent handwashing, avoiding touching the eyes, and not sharing towels or linens are the core measures. For contact lens wearers, guidance advises that patients should avoid contact lens wear altogether if unwell, particularly with any cold or flu-like symptoms. Careful cleaning of eye-care equipment and solutions is equally important in clinical settings.

School and daycare exclusion remains contested. Major guidelines take a permissive position: guidelines from the CDC and the AAP recommend against exclusion from school for most children with conjunctivitis without systemic symptoms. A 2007 survey of state health departments, by contrast, found no consensus; of 43 responding states, 13 excluded children until the disease was noncommunicable, and 16 required physician approval for return to school. Seventeen states gave multiple recommendations, which were often contradictory. Some authors argue that the literature favors exclusion until a child is asymptomatic, while others note that there is no evidence based support for exclusion or isolation of children with bacterial or viral conjunctivitis. In practice, clinicians have been unevenly informed, and a qualitative study found that no providers were aware of pediatric treatment guidelines or standardized recommendations for return to school or daycare.

Epidemiology and regional burden

Conjunctivitis is one of the most frequent eye complaints in primary care. The estimated incidence of bacterial conjunctivitis in the United States is 135 cases per 10,000 population annually, constituting approximately 1% of all primary care consultations. A Dutch national survey of general practice recorded an overall incidence rate of 13.9 per 1000 person-years, with rates above 80 per 1,000 in infants under one year. Bacterial disease is concentrated in young children: in the United States, bacterial conjunctivitis is more prevalent in children than in adults, and the 0–9 age range accounts for roughly half of cases. Etiology also varies by age group and setting. In a 2005 study of 326 children, bacterial pathogens were found in 67% of patients, viruses alone in 3%, and both bacteria and viruses in 10%.

Allergic conjunctivitis is harder to count. In the general population, the prevalence of isolated allergic conjunctivitis is 6%-30%, and reviews often cite a range of 10–30%. Much of this uncertainty reflects the fact that population-based studies estimate that 15% to 40% of the global population experiences ocular allergic symptoms during their lifetime, although the true prevalence is likely underestimated because many individuals self-medicate with over-the-counter medications. A registry analysis based on diagnostic codes in the United States produced much lower figures, suggesting a prevalence of allergic conjunctivitis of 2.8% to 3.5%, and in the same dataset the overall incidence was highest in the spring. Eye symptoms are also common among people with other allergies: ocular symptoms accompany allergic rhinitis in approximately 60% to 90% of affected individuals, while another estimate puts eye involvement at up to 40-60% of allergy sufferers.

Outbreaks follow a different pattern and are most visible in Asia. South and Southeast Asia have experienced unprecedented seasonal surges in conjunctivitis cases, with tens of thousands of cases occurring over short periods, particularly during monsoon and pre-monsoon seasons. In late 2022, a surge in Tamil Nadu, India, brought more than 100,000 patients for treatment, a count that coincided with the start of the northeast, or winter, monsoon season. A large 2002 epidemic of acute hemorrhagic conjunctivitis in South Korea reached more than a million people, and at its peak in mid-September, 1,100 schools were closed. Later clusters were linked to enterovirus 70 and Coxsackievirus A24 variant, the two enteroviruses most often implicated in these epidemics, and in autumn 2023 record cases of infectious conjunctivitis were documented in India, Nepal, Pakistan, and Vietnam. Outbreaks also occur in institutional settings, with numerous reports on outbreaks of acute bacterial conjunctivitis in day-care centres, boarding schools, military camps, nursing homes, intensive care units.

Published burden figures should be read with caution. Counts depend on how a case is defined and on whether patients seek care, and simple allergic conjunctivitis is often underrecognized, making the true prevalence difficult to determine because many patients do not seek medical care for symptoms. Outbreak investigations can also be slow to name a cause. During a 2023 outbreak in Bhutan, laboratory results from clinical specimens were initially inconclusive in identifying the etiological agent. Estimates from different countries and years therefore often reflect differences in surveillance systems as much as differences in true incidence, which is why comparisons across regions remain approximate.

Trachoma and global blindness prevention

Trachoma is a chronic eye infection caused by the bacterium Chlamydia trachomatis, which is an obligate intracellular organism. Repeated episodes of conjunctival infection in childhood produce persistent inflammation, and each episode adds to a scarring process. Over years, the scarring pulls the eyelid inward, so the lashes rub against the cornea with every blink. This stage, trachomatous trichiasis, is the one that most directly leads to blindness, and the corneal damage it causes is largely irreversible. Trachoma is therefore a disease of cumulative exposure rather than a single acute episode. Its blinding complications can appear in children in areas where infection pressure is high, which distinguishes it from the self-limited acute forms discussed elsewhere in this entry.

The scale of the problem is considerable. The WHO fact sheet describes trachoma as the leading infectious cause of blindness worldwide, responsible for about 1.4% of all blindness, and it notes that the disease remains a public health problem in part of 29 countries. The estimated global burden of trachomatous trichiasis in 2023 was 1.5 million cases, and women carry a higher relative risk, in part because they often act as caregivers for infected children. Africa remains the most affected continent. Trachoma mainly affects the poorest and most rural communities, where access to water and hygiene is limited, and the economic cost of lost productivity from blindness and visual impairment is estimated at US$2.9–5.3 billion a year.

The World Health Organization adopted the SAFE strategy in 1993 as its recommended framework for elimination. The letters stand for surgery for trichiasis, antibiotics to clear infection, facial cleanliness, and environmental improvement. Each component targets a different stage of the disease. Surgery corrects the inturned lashes that cause the blinding damage, while antibiotics, particularly mass administration of azithromycin donated through the International Trachoma Initiative, reduce the reservoir of infection in a community. Facial hygiene and improved access to water and sanitation address transmission, which occurs through contact with ocular and nasal secretions. The evidence base is uneven: systematic reviews have found strong support for surgery in reducing trichiasis and reasonably strong evidence that antibiotic distribution lowers infection rates, while evidence for the environmental and hygiene components is more limited. WHO launched the Global Elimination of Trachoma initiative (GET2020) in 1996 to coordinate country-level implementation, and its successor framework now operates within the road map for neglected tropical diseases 2021–2030.

Progress has been measurable. WHO reports that in 2025 national programmes provided corrective surgery to 98,829 people with trachomatous trichiasis and that 47.4 million people in endemic communities received antibiotics. WHO validates elimination of trachoma as a public health problem when the prevalence of trichiasis unknown to the health system is below 0.2% in people aged 15 and over, the prevalence of follicular trachomatous inflammation in children aged 1–9 is below 5% in each formerly endemic district, and a system exists to identify and manage new cases of trichiasis. As of early September 2026, WHO listed 33 countries as having been validated, with several validations in 2026 alone, including Tunisia in May and El Salvador in July. Elimination requires sustained effort, because reinfection is common where water and sanitation remain poor.

Trachoma differs from acute chlamydial conjunctivitis in several respects. The acute form is usually a single infection that may be treated and resolved, whereas trachoma reflects chronicity, repeated reinfection within the same households, and eventual scarring. Its geographic concentration in low-resource settings means that control depends as much on water supply, sanitation, and health-system reach as on drug treatment. For this reason, trachoma control programmes illustrate how the diagnosis and treatment of a common eye infection connect to broader public health and sanitation work, and why surgery, antibiotics, hygiene, and environmental change are pursued together rather than as isolated measures.

Historical understanding and treatment

Inflamed, watering eyes appear among the earliest recorded medical complaints. The first known references to inflammation of the eye can be traced back to the ancient civilisations of Egypt, Greece, Rome, Persia, China, and India. The Ebers Papyrus, compiled around 1550 BCE, devotes 8 of its 110 columns to eye problems, but it is usually very difficult to identify the diseases it describes. Egyptian practitioners believed that many eye diseases were punishments from the divine or caused by external environmental factors, and their treatments combined ointments with ritual. Some eye ointments of the period contained myrrh, resin, malachite, yellow ocher, and red natron.

Bacteriology in the late nineteenth century gave conjunctivitis its first identified bacterial cause. Koch first observed the organism in 1883, when he studied 50 patients in Egypt who were suffering from Egyptian eye disease, during the German Cholera Commission's work there. Weeks later isolated the same organism in New York, in 1886. Neither investigator named it, and Pittman and Davis (1950) characterized isolates during a field investigation of acute conjunctivitis and proposed the currently accepted nomenclature. The organism is now classified as Haemophilus aegyptius, and genomic analyses suggest that H. aegyptius is identical to H. influenzae with DNA homology of 80% to 100%. Historically, its disease followed a more severe course: a typical onset in one eye with watering, vascular injection, and pain followed by purulent discharge and bilateral involvement with significant edema of the eyelids.

Prevention of neonatal eye infection came next. In 1881, Credé introduced silver nitrate drops for newborns to prevent gonococcal ophthalmia, a method that was later diluted to 1% silver nitrate, and became a standard practice in obstetrics. One series reported that the incidence of this disease fell from 8% to 0.3% after the method was adopted. Silver nitrate had drawbacks, since it caused transient chemical conjunctivitis in 50% to 90% of infants. Later prophylaxis relied on other agents, including erythromycin, tetracycline, and gentamicin. Once prenatal screening became routine, neonatal gonococcal and chlamydial cases became less common in some settings, and Chlamydia, which is now the most common cause of ophthalmia neonatorum in developed countries, drew increasing attention.

Modern understanding of viral and allergic forms developed in the twentieth century. Adenoviruses were recovered in the early 1950s from cultured adenoid tissue, which gave the group its name and clarified the viral basis of epidemic eye infections. In the 1960s, research groups in Japan and Sweden identified IgE as a distinct antibody class, and it became clear that IgE bound to mast cells triggers the release of histamine on exposure to an allergen. This mechanism reshaped how allergic conjunctivitis was classified and how it was treated, and it underpinned later antihistamine and mast cell stabilizer therapy.

Open questions and research directions

Culture has long been the reference standard for identifying the organisms behind infectious conjunctivitis and related eye infections, yet a meaningful share of specimens do not grow anything. Culture-independent methods are being tested to fill this gap. In a small nanopore sequencing study of endophthalmitis, an intraocular infection rather than a surface one, 18 of 23 patients were culture-positive, and whole-genome sequencing identified potential pathogens in two culture-negative cases. One of those cases contained sequencing-detected Cutibacterium acnes, a skin commensal, alongside another bacterium. Similar findings have appeared in other culture-negative infections. A 16S-based study of culture-negative clinical samples detected bacteria in a fraction of specimens and found C. acnes to be the most frequent organism, but it also left about a quarter of samples negative by both culture and sequencing.

The difficulty is interpreting these results. C. acnes is slow-growing, prone to biofilm formation, and easily introduced through contamination during sampling, so a detected read does not by itself show that the organism caused disease. Comparative work with anaerobic culture suggests that a sequencing-negative, culture-positive result most likely reflects a low bacterial load, and that cultures held beyond about seven days add little. Investigators have also noted that prolonged incubation may increase the chance that a late positive culture reflects contamination. Causation therefore depends on clinical context, repeated sampling and corroborating findings, and no single laboratory result settles it.

Rapid point-of-care tests are being developed to shorten the gap between presentation and an informed treatment decision. This matters because a large share of conjunctivitis cases in primary care receive antibiotics, even though most acute cases are viral and self-limiting. In one evaluation of a commercial adenovirus immunoassay, 186 adults with presumed infectious conjunctivitis of four days or less were tested. Of 56 eyes that were positive on the immunoassay, only 28 were confirmed by quantitative PCR, a positive predictive value of 50%. The same group examined whether the signal intensity of the test bands affected accuracy. Accuracy is also population-dependent: in a large US series, only about 8.6% of patients suspected of adenoviral disease tested positive by PCR, which means that a test with modest predictive value in one setting can perform quite differently in another. Stewardship benefits therefore depend on local prevalence, and on whether results are interpreted together with clinical signs rather than in isolation.

Antiviral treatment remains the clearest unmet need in the adenoviral disease that accounts for much of the viral burden. No antiviral has FDA approval for human adenovirus infection, and acute care is largely supportive. Topical ganciclovir gel is used off label and has shown some efficacy against certain serotypes, while reviews have found that many agents work in cell culture and animal models but lack reliable benefit in large controlled trials. Oral antivirals have not been shown to affect adenoviral ocular disease. Povidone-iodine and topical corticosteroids reduce symptom duration and complications in some settings, and combination products containing povidone-iodine and low-dose dexamethasone have reached phase III trials. Persistence of the virus after the active phase, and the recurrence of subepithelial infiltrates, make it likely that research into viral inhibitors and better-tolerated combinations will remain active. Design challenges, including the self-limiting course and variable serotype susceptibility, have made it difficult to produce definitive evidence.

Chronic and severe allergic forms present a different kind of open question. The IgE and mast cell pathway that produces the itch, redness and swelling of allergic conjunctivitis is well characterized, but the reasons some individuals progress to vernal or atopic disease with corneal involvement, and why responses to allergen immunotherapy vary, are less clearly understood. Without a fuller explanation of these processes, long-term management continues to rely largely on symptom control, and more targeted therapies await clearer mechanistic targets. Together, the unresolved questions in diagnosis, antiviral therapy and allergic pathogenesis show how much of conjunctivitis care still depends on clinical judgment and on the limits of current laboratory and therapeutic tools.

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