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

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.

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.

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.

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.

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.

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.

Allergic conjunctivitis is inflammation of the conjunctiva (the membrane covering the white part of the eye) due to allergy. Although allergens differ among patients, the most common cause is hay fever. Symptoms consist of redness (mainly due to vasodilation of the peripheral small blood vessels), edema (swelling) of the conjunctiva, itching, and increased lacrimation (production of tears). If this is combined with rhinitis, the condition is termed allergic rhinoconjunctivitis.

The symptoms are due to release of histamine and other active substances by mast cells, which stimulate dilation of blood vessels, irritate nerve endings, and increase secretion of tears.

Treatment of allergic conjunctivitis is by avoiding the allergen ("e.g.", avoiding grass in bloom during "hay fever season") and treatment with antihistamines, either topical (in the form of eye drops), or systemic (in the form of tablets). Antihistamines, medications that stabilize mast cells, and nonsteroidal anti-inflammatory drugs (NSAIDs) are generally safe and usually effective.

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.

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.

Histopathological findings from affected humans indicate that wound healing is impaired due to a deficiency in plasmin-mediated extracellular fibrinolysis. Episodes may be triggered by minor trauma, eye surgery, or by systemic events such as infections or antifibrinolytic therapy. Histology shows amorphous subepithelial deposits of eosinophilic material consisting predominantly of fibrin.

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.

Ligneous conjunctivitis may be managed by topical treatments of plasminogen, topical and subconjunctival fresh frozen plasma, and fibrinolytic therapy.

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.

Herpetic simplex keratitis, also known as herpetic keratoconjunctivitis and herpesviral keratitis, is a form of keratitis caused by recurrent herpes simplex virus (HSV) infection in the cornea.

It begins with infection of epithelial cells on the surface of the eye and retrograde infection of nerves serving the cornea. Primary infection typically presents as swelling of the conjunctiva and eyelids (blepharoconjunctivitis), accompanied by small white itchy lesions on the corneal surface. The effect of the lesions varies, from minor damage to the epithelium (superficial punctate keratitis), to more serious consequences such as the formation of dendritic ulcers. Infection is unilateral, affecting one eye at a time. Additional symptoms include dull pain deep inside the eye, mild to acute dryness, and sinusitis. Most primary infections resolve spontaneously in a few weeks. Healing can be aided by the use of oral and topical antivirals.

Subsequent recurrences may be more severe, with infected epithelial cells showing larger dendritic ulceration, and lesions forming white plaques. The epithelial layer is sloughed off as the dendritic ulcer grows, and mild inflammation (iritis) may occur in the underlying stroma of iris. Sensation loss occurs in lesional areas, producing generalised corneal anaesthesia with repeated recurrences. Recurrence can be accompanied by chronic dry eye, low grade intermittent conjunctivitis, or chronic unexplained sinusitis. Following persistent infection the concentration of viral DNA reaches a critical limit. Antibody responses against the viral antigen expression in the stroma can trigger a massive immune response in the eye. The response may result in the destruction of the corneal stroma, resulting in loss of vision due to opacification of the cornea. This is known as immune-mediated stromal keratitis.

HSV infection is very common in humans. It has been estimated that one third of the world population have recurrent infection. Keratitis caused by HSV is the most common cause of cornea-derived blindness in developed nations. Therefore, HSV infections are a large and worldwide public health problem. The global incidence (rate of new disease) of herpes keratitis is roughly 1.5 million, including 40,000 new cases of severe monocular visual impairment or blindness each year.

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.

Most of the time, scleritis is not caused by an infectious agent. Histopathological changes are that of a chronic granulomatous disorder, characterized by fibrinoid necrosis, infiltration by polymorphonuclear cells, lymphocytes, plasma cells and macrophages. The granuloma is surrounded by multinucleated epitheloid giant cells and new vessels, some of which may show evidence of vasculitis.

Diffuse lamellar keratitis (DLK) is a sterile inflammation of the cornea which may occur after refractive surgery, such as LASIK. Its incidence has been estimated to be 1 in 500 patients, though this may be as high as 32% in some cases.

Patients typically present within one week of surgery with eye pain, photophobia, conjunctivitis, or excessive tear production.

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.

Chronic conjunctivitis (e.g. trachoma) and aging factor are two causes of conjunctival concretion, which will make the conjunctiva cellular degeneration to produce an epithelial inclusion cyst, filled with epithelial cells and keratin debris. After calcification, the conjunctival cyst hardens and forms a conjunctival concretion. Congenital conjunctival concretion condition is also more common.

Conjunctival concretions can be single, also multiple, less confluent. There is no difference between the site of the occurrence on the upper and lower eyelid, nor right or left eye. The vast majority of concretions are in the conjunctival surface rather than deep. There is no difference in age for predilection or incidence of concretions, due to the causes of conjunctivitis, aging, and even congenital factor.

Parinaud's oculoglandular syndrome is the combination of granulomatous conjunctivitis in one eye, and swollen lymph nodes in front of the ear on the same side. Most cases are caused by cat-scratch disease, although it is an unusual feature of this condition. Occasionally it may be caused by other infections.

It should not be confused with the neurological syndrome caused by a lesion in the midbrain which is also known as Parinaud's syndrome. Both were named after the same person, Henri Parinaud.

Causes include:

- "Bartonella henselae"

- "Francisella tularensis"

- herpes simplex virus type 1

- "Paracoccidioides brasiliensis"

FVR is transmitted through direct contact only. It replicates in the nasal and nasopharyngeal tissues and the tonsils. Viremia (the presence of the virus in the blood) is rare. The virus is shed in saliva and eye and nasal secretions, and can also be spread by fomites. FVR has a two- to five-day incubation period. The virus is shed for one to three weeks postinfection. Latently infected cats (carriers) will shed FHV-1 intermittently for life, with the virus persisting within the trigeminal ganglion. Stress and use of corticosteroids precipitate shedding. Most disinfectants, antiseptics and detergents are effective against the virus.

There are various causes of madarosis.

- Ophthalmological conditions: blepharitis is an infection of the eyelid. Anterior blepharitis is either "staphylococcal blepharitis,"or "seborrhoeic blepharitis" and posterior blepharitis is due to the meibomian gland.

- Dermatologic conditions: there are multiple types of dermatological conditions that can result in madarosis. These include Atopic dermatitis, Seborrhoeic dermatitis atopic dermatitis, and Psoriasis on the eyelids can result in madarosis. Others include: frontal fibrosing alopecia, ulerythema ophryogenes, acne rosacea, telogen effluvium, follicular mucinosis, and cutaneous sarcoidosis.

- Nutritional defects: Severe malnutrition can cause chronic hair loss. Hypoproteinemia causes hair loss by early onset of telogen. Zinc deficiencies like acrodermatitis enteropathica, can lead to the loss of eyebrow/eyelash hair. Other deficiencies like biotin and iron make it possible for loss of hair as well.

- Infections: There are many bodily infections that can cause the loss of eyelashes/eyebrows. The most common infection may be leprosy, such as lepromatous leprosy. Syphilis or other viral infections like herpes or HIV can cause the loss of eye hair as well. Fungal infections, like paracoccidioidomycosis, trichophyton, or microsporum, are also possible infection causes.

- Trauma: Most trauma injuries cause madarosis from the psychological standpoint, known as trichotillomania

- Drugs/Medications: Crack cocaine or chemotherapy drugs. Other drugs include:propranolol, valproic acid, barbiturates, MMR vaccine, botulinum toxin, epinephrine, antithyroid drugs, anticoagulants, and lipid-lowering drugs

- Genetics

- Autoimmune disorders: alopecia areata, discoid lupus erythematosus, chronic cutaneous lupus erythmatosus, Graham-Little syndrome, and Parry Romberg syndrome

- Other diseases: hypothyroidism, hyperthyroidism, hypoparathyroidism, hypopituitarism, and amyloidosis

There only prevention method is determining the underlying condition before treatment options are too late.