Uveitis is usually an isolated illness, but can be associated with many other medical conditions.

In anterior uveitis, no associated condition or syndrome is found in approximately one-half of cases. However, anterior uveitis is often one of the syndromes associated with HLA-B27. Presence of this type of HLA allele has a relative risk of evolving this disease by approximately 15%.

The most common form of uveitis is acute anterior uveitis (AAU). It is most commonly associated with HLA-B27, which has important features: HLA-B27 AAU can be associated with ocular inflammation alone or in association with systemic disease. HLA-B27 AAU has characteristic clinical features including male preponderance, unilateral alternating acute onset, a non-granulomatous appearance, and frequent recurrences, whereas HLA-B27 negative AAU has an equivalent male to female onset, bilateral chronic course, and more frequent granulomatous appearance. Rheumatoid arthritis is not uncommon in Asian countries as a significant association of uveitis.

Uveitis may be an immune response to fight an infection inside the eye. While representing the minority of patients with uveitis, such possible infections include:

- brucellosis

- leptospirosis

- Lyme disease

- presumed ocular histoplasmosis syndrome

- syphilis

- toxocariasis

- toxoplasmic chorioretinitis

- tuberculosis

- Zika fever

Although intermediate uveitis can develop at any age, it primarily afflicts children and young adults. There is a bimodal distribution with one peak in the second decade and another peak in the third or fourth decade.

In the United States the proportion of patients with intermediate uveitis is estimated to be 4-8% of uveitis cases in referral centers. The National Institutes of Health reports a higher percentage (15%), which may indicate improved awareness or the nature of the uveitis referral clinic. In the pediatric population, intermediate uveitis can account for up to 25% of uveitis cases.

Pars planitis is considered a subset of intermediate uveitis and is characterized by the presence of white exudates (snowbanks) over the pars plana or by aggregates of inflammatory cells in the vitreous (snowballs) in the absence of an infectious or a systemic disease. Some physicians believe that patients with pars planitis have worse vitritis, more severe macular edema, and a guarded prognosis compared to other patients with intermediate uveitis.

Episcleritis is a common disease, and its exact prevalence and incidence are unknown. It typically affects young adults, and may be more common in women.

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.

Episcleritis is a benign, self-limiting condition, meaning patients recover without any treatment. Most cases of episcleritis resolve within 7–10 days. The nodular type is more aggressive and takes longer to resolve. Although rare, some cases may progress to scleritis. However, in general, episcleritis does not cause complications in the eye. Smoking tobacco delays the response to treatment in patients with episcleritis.

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.

Sympathetic ophthalmia is rare, affecting 0.2% to 0.5% of non-surgical eye wounds, and less than 0.01% of surgical penetrating eye wounds. There are no gender or racial differences in incidence of SO.

ARN is associated with people who have latent herpes viruses that have been reactivated. The most common causes of the disease have been linked to VSV, HSV-1, HSV-2, and CMV respectively.

ARN cases have been reported in patients who have AIDS, are immunocompromised and in children. The disease is not limited to a specific gender. Most cases have been reported in young adults though children and the elderly can be affected.

Specific genetic markers in Caucasians in the United States have shown elevated risk for disease development (HLA-DQw7 and Bw62, DR4) as well as HLA-Aw33, B44, and DRw6 in the Japanese population.

Birdshot chorioretinopathy is a rare form of posterior uveitis and accounts for 1-3% of uveitis cases in general. Birdshot chorioretinopathy is thought to be an autoimmune disease. The disease has strong association with the Human leukocyte antigen haplotype (HLA)-A29, which is the strongest association between a disease and HLA class I documented (>99% of patients are HLA-A29 positive by molecular testing and HLA-A29-negative cases are controversial). This indicates a role for T-lymphocytes in the pathogenesis. Birdshot chorioretinopathy is associated with IL-17, a hallmark cytokine of TH17 cells that play an important role in autoimmunity . The disease affects typically middle-aged or elderly caucasians. HLA-A29 is less prevalent in Asia and no birdshot chorioretinopathy cases have been reported in Asia. When birdshot chorioretinopathy is suspected, a person is usually tested to determine if they are HLA-A29 positive. Although previously HLA-A29 testing was not considered necessary for definitive diagnosis, because HLA-A29 is also common in the general healthy population (7%). An increasing number of specialists consider the presence of HLA-A29 critical for diagnosis. Additional (genetic or environmental) or unknown factors may be associated with HLA-A29 in the pathogenesis.

In 2014, Kuiper et al. conducted a genome-wide association study in birdshot chorioretinopathy and studied the entire genome of Dutch, Spanish and English patients. This large genetic study ascertained HLA-A29:02 as the primary risk factor and identified the "endoplasmic reticulum aminopeptidase (ERAP) 2" gene strongly associated with birdshot chorioretinopathy. Genetic variants near "ERAP2" on chromosome 5 resulted in high mRNA and protein expression of this aminopeptidase in BSCR patients. ERAP2 is an aminopeptidase that, together with the closely related ERAP1, trims peptides in the endoplasmic reticulum and loads these peptides on HLA molecules for presentation to T cells of the immune system. ERAP-HLA associations have also been reported in Ankylosing spondylitis and Behcet's disease, suggesting shared pathogenic pathways among these diseases.

Affected individuals are typically 20 to 50 years old. The female to male ratio is 2:1. By definition, there is no history of either surgical or accidental ocular trauma. VKH is more common in Asians, Latinos, Middle Easterners, American Indians, and Mexican Mestizos; it is much less common in Caucasians and in blacks from sub-Saharan Africa.

VKH is associated with a variety of genetic polymorphisms that relate to immune function. For example, VKH has been associated with human leukocyte antigens (HLA) HLA-DR4 and DRB1/DQA1, copy-number variations (CNV) of complement component 4, a variant IL-23R locus and with various other non-HLA genes. HLA-DRB1*0405 in particular appears to play an important susceptibility role.

Although there is sometimes a preceding viral infection, or skin or eye trauma, the exact underlying initiator of VKH disease remains unknown. However, VKH is attributed to aberrant T-cell-mediated immune response directed against self-antigens found on melanocytes. Stimulated by interleukin 23 (IL-23), T helper 17 cells and cytokines such as interleukin 17 (IL-17) appear to target proteins in the melanocyte.

In a study done published by the British Journal of Ophthalmology, the cases of ARN/BARN reported in 2001-2002 in the UK, Varicella Zoster Virus was the most common culprit for the disease and presented mostly in men than in women.

Researchers have also looked at two cases of ARN in patients who have been diagnosed with an immunodeficiency virus. The disease presented itself more so in the outer retina until it progressed far enough to then affect the inner retina. The patients were not so responsive to the antiviral agents given to them through an IV, acyclovir specifically. The cases progressed to retinal detachment. The patients tested positive for the herpes virus. Researchers are now wondering if this type of ARN is specific to those who have the immunodeficiency virus.

Sympathetic ophthalmia is currently thought to be an autoimmune inflammatory response toward ocular antigens, specifically a delayed hypersensitivity to melanin-containing structures from the outer segments of the photoreceptor layer of the retina. The immune system, which normally is not exposed to ocular proteins, is introduced to the contents of the eye following traumatic injury. Once exposed, it senses these antigens as foreign, and begins attacking them. The onset of this process can be from days to years after the inciting traumatic event.

It is a very rare disease. Approximately 200 cases were reported in medical journals in the 35 years after its initial description. Altogether, more than 100 cases have been reported in Japan.

Chorioretinitis is usually treated with a combination of corticosteroids and antibiotics. However, if there is an underlying cause such as HIV, specific therapy can be started as well.

A 2012 Cochrane Review found weak evidence suggesting that ivermectin could result in reduced chorioretinal lesions in patients with onchocercal eye disease. More research is needed to support this finding.

Birdshot chorioretinopathy now commonly named "Birdshot Uveitis" or ""HLA-A29 Uveitis"" is a rare form of bilateral posterior uveitis affecting the eye. It causes severe, progressive inflammation of both the choroid and retina.

Affected individuals are almost exclusively caucasian and usually diagnosed in the fourth to sixth decade of their lives.

Chorioretinitis is often caused by toxoplasmosis and cytomegalovirus infections (mostly seen in immunodeficient subjects such as people with HIV or on immunosuppressant drugs). Congenital toxoplasmosis via transplacental transmission can also lead to sequelae such as chorioretinitis along with hydrocephalus and cerebral calcifications. Other possible causes of chorioretinitis are syphilis, sarcoidosis, tuberculosis, Behcet's disease, onchocerciasis, or West Nile virus. Chorioretinitis may also occur in presumed ocular histoplasmosis syndrome (POHS); despite its name, the relationship of POHS to "Histoplasma" is controversial.

Hypopyon can be present in a corneal ulcer. Behcet's disease, endophthalmitis, panuveitis/panophthalmitis and adverse reactions to some drugs (such as rifabutin).

Hypopyon is also known as "sterile pus", as it occurs due to the release of toxins and not by the actual invasion of pathogens. The toxins secreted by the pathogens mediate the outpouring of leukocytes that settle in the anterior chamber of the eye. .

An inverse hypopyon is different from a standard hypopyon. Inverse hypopyon is seen after a pars plana vitrectomy with an insertion of silicone oil (as a replacement of the vitreous humour that has been removed in the operation; the silicone oil maintains internal tamponade). When the silicone oil emulsifies, it seeps into the anterior chamber and settles at the top of the anterior chamber. This is in contrast to hypopyon resulting from toxins where the leukocytes settle at the bottom of the anterior chamber. This is due to the effect of gravity, hence the name inverse hypopyon.

IOI or orbital pseudotumor is the second most common cause of exophthalmos following Grave’s orbitopathy and the third most common orbital disorder following thyroid orbitopathy and lymphoproliferative disease accounting for 5–17.6% of orbital disorders, There is no age, sex, or race predilection, but it is most frequently seen in middle-aged individuals. Pediatric cases account for about 17% of all cases of IOI.

Hypopyon is a medical condition involving inflammatory cells in the anterior chamber of the eye.

It is a leukocytic exudate, seen in the anterior chamber, usually accompanied by redness of the conjunctiva and the underlying episclera. It is a sign of inflammation of the anterior uvea and iris, i.e. iritis, which is a form of anterior uveitis. The exudate settles at the dependent aspect of the eye due to gravity. It can be sterile (in bacterial corneal ulcer) or not sterile (fungal corneal ulcer).

The exact cause of IOI is unknown, but infectious and immune-mediated mechanisms have been proposed. Several studies have described cases where onset of orbital pseudotumor was seen simultaneously or several weeks after upper respiratory infections. Another study by Wirostko et al. proposes that organisms resembling Mollicutes cause orbital inflammation by destroying the cytoplasmic organelles of parasitized cells.

Orbital pseudotumor has also been observed in association with Crohn’s disease, systemic lupus erythematosus, rheumatoid arthritis, diabetes mellitus, myasthenia gravis, and ankylosing spondylitis all of which strengthen the basis of IOI being an immune-mediated disease. Response to corticosteroid treatment and immunosuppressive agents also support this idea.

Trauma has also been seen to precede some cases of orbital pseudotumor. However, one study by Mottow-Lippe, Jakobiec, and Smith suggests that the release of circulating antigens caused by local vascular permeability triggers an inflammatory cascade in the affected tissues.

Although these mechanisms have been postulated as possible causes of IOI, their exact nature and relationships to the condition still remain unclear.

Uveitis may cause pain of the affected eye together with changes in vision. It may be accompanied by nonspecific systemic symptoms such as fever, involuntary weight loss, fatigue, loss of appetite, abdominal pain, and joint pains.

With Behçet's disease as an intercurrent disease in pregnancy, the pregnancy does not have an adverse effect on the course of Behçet's disease and may possibly ameliorate its course. Still, there is a substantial variability in clinical course between patients and even for different pregnancies in the same patient. Also, the other way around, Behçet's disease confers an increased risk of pregnancy complications, miscarriage and Cesarean section.

Behçet's can cause male infertility, either as a result of the condition itself or of a side effect of concomitant medication such as Colchicine, which is known to lower sperm count.