Adenoviruses is the most common cause of viral conjunctivitis (adenoviral keratoconjunctivitis). Herpetic keratoconjunctivitis (caused by herpes simplex viruses) can be serious and requires treatment with acyclovir. Acute hemorrhagic conjunctivitis is a highly contagious disease caused by one of two enteroviruses, Enterovirus 70 and Coxsackievirus A24. These were first identified in an outbreak in Ghana in 1969, and have spread worldwide since then, causing several epidemics.

Conjunctivitis may also be caused by allergens such as pollen, perfumes, cosmetics, smoke, dust mites, Balsam of Peru, and eye drops.

The cause of allergic conjunctivitis is an allergic reaction of the body's immune system to an allergen. Allergic conjunctivitis is common in people who have other signs of allergic disease such as hay fever, asthma and eczema.

Among the most common allergens that cause conjunctivitis are:

- Pollen from trees, grass and ragweed

- Animal skin and secretions such as saliva

- Perfumes

- Cosmetics

- Skin medicines

- Air pollution

- Smoke

- Dust mites

- Balsam of Peru (used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in medicine and pharmaceutical items for healing properties)

- Eye drops

Most cases of seasonal conjunctivitis are due to pollen and occur in the hay fever season, grass pollens in early summer and various other pollens and moulds may cause symptoms later in the summer.

Keratoconjunctivitis is inflammation ("-itis") of the cornea and conjunctiva.

When only the cornea is inflamed, it is called "keratitis"; when only the conjunctiva is inflamed, it is called "conjunctivitis".

There are several potential causes of the inflammation:

- Keratoconjunctivitis sicca is used when the inflammation is due to dryness. ("Sicca" means "dryness" in medical contexts.) It occurs with 20% of rheumatoid arthritis patients.

- The term "Vernal keratoconjunctivitis" (VKC) is used to refer to keratoconjunctivitis occurring in spring, and is usually considered to be due to allergens.

- "Atopic keratoconjunctivitis" is one manifestation of atopy.

- "Epidemic keratoconjunctivitis" is caused by an adenovirus infection.

- "Infectious bovine keratoconjunctivitis" (IBK) is a disease affecting cattle caused by the bacteria "Moraxella bovis".

- "Pink eye in sheep and goat" is another infectious keratoconjunctivitis of veterinary concern, mostly caused by "Chlamydophila pecorum"

- "Superior limbic keratoconjunctivitis" is thought to be caused by mechanical trauma.

- "Keratoconjunctivitis photoelectrica" (arc eye) means inflammation caused by photoelectric UV light. It is a type of ultraviolet keratitis. Such UV exposure can be caused by arc welding without wearing protective eye glass, or by high altitude exposure from sunlight reflected from snow ("snow blindness"). The inflammation will only appear after about 6 to 12 hours. It can be treated by rest, as the inflammation usually heals after 24–48 hours. Proper eye protection should be worn to prevent keratoconjunctivitis photoelectrica.

Several aetiologies are suggested, and any combination of these may be present in any given case.

- Vitamin deficiency (A, B or C)

- Viral infection

- Bacterial infection

- "Leptospira

- "Streptococcus

- "Brucella

- Parasitic infection

- Strongyle

- "Onchocerca cervicalis"

- Autoimmune disease

The disease has been suggested to be primarily autoimmune in nature, being a delayed hypersensitivity reaction to any of the above agents.

The Appaloosa has a higher risk of developing ERU than other breeds; this predisposition has a genetic basis. Appaloosas which develop ERU are more likely than other breeds to have ERU in both eyes, and more likely to become blind in one or both eyes.

One way to prevent allergic rhinitis is to wear a respirator or mask when near potential allergens.

Growing up on a farm and having multiple brothers and or sisters decreases the risk.

The disease incidence varies widely depending on the geographical location. The most extensive epidemiological survey on this subject has been carried out by Dharmasena et al. who analysed the number of neonates who developed neonatal conjunctivitis in England from 2000 to 2011. In addition to the incidence of this sight threatening infection they also investigated the time trends of the disease. According to them the incidence of Neonatal conjunctivitis (Ophthalmia Neonatorum) in England was 257 (95% confidence interval: 245 to 269) per 100,000 in 2011.

PKC results from a delayed hypersensitivity/inflammatory reaction to antigens expressed by various pathogens. Common agents include Staph. aureus, Mycobacterium tuberculosis, Chlamydia and Candida.

Allergic rhinitis triggered by the pollens of specific seasonal plants is commonly known as "hay fever", because it is most prevalent during haying season. However, it is possible to have allergic rhinitis throughout the year. The pollen that causes hay fever varies between individuals and from region to region; in general, the tiny, hardly visible pollens of wind-pollinated plants are the predominant cause. Pollens of insect-pollinated plants are too large to remain airborne and pose no risk. Examples of plants commonly responsible for hay fever include:

- Trees: such as pine ("Pinus"), birch ("Betula"), alder ("Alnus"), cedar ("Cedrus"), hazel ("Corylus"), hornbeam ("Carpinus"), horse chestnut ("Aesculus"), willow ("Salix"), poplar ("Populus"), plane ("Platanus"), linden/lime ("Tilia"), and olive ("Olea"). In northern latitudes, birch is considered to be the most common allergenic tree pollen, with an estimated 15–20% of people with hay fever sensitive to birch pollen grains. A major antigen in these is a protein called Bet V I. Olive pollen is most predominant in Mediterranean regions. Hay fever in Japan is caused primarily by sugi ("Cryptomeria japonica") and hinoki ("Chamaecyparis obtusa") tree pollen.

- "Allergy friendly" trees include: ash (female only), red maple, yellow poplar, dogwood, magnolia, double-flowered cherry, fir, spruce, and flowering plum.

- Grasses (Family Poaceae): especially ryegrass ("Lolium" sp.) and timothy ("Phleum pratense"). An estimated 90% of people with hay fever are allergic to grass pollen.

- Weeds: ragweed ("Ambrosia"), plantain ("Plantago"), nettle/parietaria (Urticaceae), mugwort ("Artemisia Vulgaris"), Fat hen ("Chenopodium"), and sorrel/dock ("Rumex")

Allergic rhinitis may also be caused by allergy to Balsam of Peru, which is in various fragrances and other products.

Chemical irritants such as silver nitrate can cause chemical conjunctivitis, usually lasting 2–4 days. Thus, prophylaxis with a 1% silver nitrate solution is no longer in common use. In most countries neomycin and chloramphenicol eye drops are used instead. However, it is possible for newborns to suffer from neonatal conjunctivitis due to reactions with chemicals in these common eye drops. Additionally, a blocked tear duct may be another non-infectious cause of neonatal conjunctivitis.

The risk factors associated with BPF are not well known. However, it has been suggested that children under 5 years of age are more susceptible to BPF since they lack serum bactericidal activity against the infection. Older children and adults have much higher titers of bactericidal antibodies, which serve as a protective measure. Also children residing in warmer geographic areas have been associated with a higher risk of BPF infection.

A drug-resistant strain of scarlet fever, resistant to macrolide antibiotics such as erythromycin, but retaining drug-sensitivity to beta-lactam antibiotics such as penicillin, emerged in Hong Kong in 2011, accounting for at least two deaths in that city—the first such in over a decade. About 60% of circulating strains of the group A "Streptococcus" which cause scarlet fever in Hong Kong are resistant to macrolide antibiotics, says Professor Kwok-yung Yuen, head of Hong Kong University's microbiology department. Previously, observed resistance rates had been 10–30%; the increase is likely the result of overuse of macrolide antibiotics in recent years.

A study conducted in November of 2017, conveyed a correlation between blepharitis and early onset metabolic syndrome (MetS). To investigate the relationship between blepharitis and MetS, researchers used the Longitudinal Health Insurance Database in Taiwan. Results indicated that hyperlipidaemia and coronary artery disease were significantly correlated with the prior development of blepharitis. Therefore, blepharitis was shown to be significantly related to MetS and can serve as an early indication of the condition.

In another recent study, the presence of Demodex has been unveiled as a common cause of blepharitis. However, the pathogenesis of demodicosis is still unclear. In this study, researchers provide a diagnosis of the disease and propose diagnostic criteria of Demodex blepharitis.

It is usually caused by allergies or viral infections, often inciting excessive eye rubbing. Chemosis is also included in the Chandler Classification system of orbital infections.

If chemosis has occurred due to excessive rubbing of the eye, the first aid to be given is a cold water wash for eyes.

Other causes of chemosis include:

- Superior vena cava obstruction, accompanied by facial oedema

- Hyperthyroidism, associated with exophthalmos, periorbital puffiness, lid retraction, and lid lag

- Cavernous sinus thrombosis, associated with infection of the paranasal sinuses, proptosis, periorbital oedema, retinal haemorrhages, papilledema, extraocular movement abnormalities, and trigeminal nerve sensory loss

- Carotid-cavernous fistula - classic triad of chemosis, pulsatile proptosis, and ocular bruit

- Cluster headache

- Trichinellosis

- Systemic lupus erythematosus (SLE)

- Angioedema

- Acute glaucoma

- Panophthalmitis

- Orbital cellulitis

- Gonorrheal conjunctivitis

- Dacryocystitis

- Spitting cobra venom to the eye

- High concentrations of phenacyl chloride in chemical mace spray

- Urticaria

- Trauma

- Post surgical

- Rhabdomyosarcoma of the orbit

Scarlet fever occurs equally in both males and females. Children are most commonly infected, typically between 5–15 years old. Although streptococcal infections can happen at any time of year, infection rates peak in the winter and spring months, typically in colder climates.

The morbidity and mortality of scarlet fever has declined since the 18th and 19th century when there were epidemics caused by this disease. Around 1900 the mortality rate in multiple places reached 25%. In The improvement in prognosis can be attributed to the use of penicillin in the treatment of this disease. The frequency of scarlet fever cases has also been declining over the past century however, there have been several reported outbreaks of the disease in various countries in the past decade. The reason for these recent increases remains unclear in the medical community.

The mechanism by which the bacteria causes symptoms of blepharitis is not fully understood and may include direct irritation of bacterial toxins and/or enhanced cell-mediated immunity to S. aureus.

Staphylococcal blepharitis is caused by an infection of the anterior portion of the eyelid by Staphylococcal bacteria. In a study of ocular flora, 46% to 51% of those diagnosed with staphylococcal blepharitis had cultures positive for Staphylococcus aureus in comparison to 8% of normal patients. Staphylococcal blepharitis may start in childhood and continue into adulthood. It is commonly recurrent and it requires special medical care. The prevalence of Staphylococcus aureus in the conjunctival sac and on the lid margin varies among countries, likely due to differences in climate and environment. Seborrheic blepharitis is characterized by less inflammation than Staphylococcal blepharitis; however, it causes more excess oil or greasy scaling. Meibomian Gland Dysfunction is a result of abnormalities of the meibomian glands and altered secretion meibum, which plays an imperative role in lagging the evaporation of tear films and smoothing of the tear film to produce an even optical surface. Posterior blepharitis is an inflammation of the eyelids, secondary to dysfunction of the meibomian glands. Like anterior blepharitis, it is a bilateral chronic condition and may be associated with skin rosacea. There is growing evidence that, in some cases, it is caused by Demodex mites.

Scleritis is not a common disease, although the exact prevalence and incidence are unknown. It is somewhat more common in women, and is most common in the fourth to sixth decades of life.

Conjunctivitis is prevalent among children of the highlands of Ecuador. The finding supports the hypothesis that prolonged exposure to the sun at altitude—in the less dense atmosphere (with the resultant lower UV absorption)—is the main cause of the disease.

Ophthalmia (also called ophthalmitis) is inflammation of the eye. It is a medical sign which may be indicative of various conditions, including sympathetic ophthalmia (inflammation of both eyes following trauma to one eye), gonococcal ophthalmia, trachoma or "Egyptian" ophthalmia, ophthalmia neonatorum (a conjunctivitis of the newborn due to either of the two previous pathogens), photophthalmia and actinic conjunctivitis (inflammation resulting from prolonged exposure to ultraviolet rays), and others.

Actinic conjunctivitis is an inflammation of the eye contracted from prolonged exposure to actinic (ultraviolet) rays. Symptoms are redness and swelling of the eyes. Most often the condition is caused by prolonged exposure to Klieg lights, therapeutic lamps, or acetylene torches. Other names for the condition include Klieg conjunctivitis, eyeburn, arc-flash, welder's conjunctivitis, flash keratoconjunctivitis, actinic ray ophthalmia, x-ray ophthalmia, and ultraviolet ray ophthalmia.

Epithelial keratitis is treated with topical antivirals, which are very effective with low incidence of resistance. Treatment of the disease with topical antivirals generally should be continued for 10–14 days. Aciclovir ophthalmic ointment and Trifluridine eye drops have similar effectiveness but are more effective than Idoxuridine and Vidarabine eye drops. Oral acyclovir is as effective as topical antivirals for treating epithelial keratitis, and it has the advantage of no eye surface toxicity. For this reason, oral therapy is preferred by some ophthalmologists.

Ganciclovir and brivudine treatments were found to be equally as effective as acyclovir in a systematic review.

Valacyclovir, a pro-drug of acyclovir likely to be just as effective for ocular disease, can cause thrombotic thrombocytopenic purpura/Hemolytic-uremic syndrome in severely immunocompromised patients such as those with AIDS; thus, it must be used with caution if the immune status is unknown.

Topical corticosteroids are contraindicated in the presence of active herpetic epithelial keratitis; patients with this disease who are using systemic corticosteroids for other indications should be treated aggressively with systemic antiviral therapy.

The effect of interferon with an antiviral agent or an antiviral agent with debridement needs further assessment.

Primary infection most commonly manifests as blepharoconjunctivitis i.e. infection of lids and conjunctiva that heals without scarring. Lid vesicles and conjunctivitis are seen in primary infection. Corneal involvement is rarely seen in primary infection.

There are three main viruses that have been studied and confirmed as the agents responsible for AHC, including enterovirus 70, coxsakievirus A24 variant (CA24v) and adenovirus 11.

AHC can only exist in a human host and is transmitted through human contact with an infected individual or object, such as a towel used by an infected person. It is also easily communicable through fecal-oral pathways, thus allowing for its abundance in areas of the world with low levels of hygiene. Within one to two days of infection, symptoms will begin to become apparent.

"Other infections include:"

- "Closed-space infections of the fingertips, known as paronychia."