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.

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.

The syndrome is rare in the United States, Africa and South America, but is common in the Middle East and Asia, suggesting a possible cause endemic to those tropical areas. A theory suggested that past exposure to lethal infectious agents might have fixed the genetic susceptibility factors to Behçet's disease in those area. It is not associated with cancer, and links with tissue-types (which are under investigation) are not certain. It also does not follow the usual pattern for autoimmune diseases. However, one study has revealed a possible connection to food allergies, particularly to dairy products. An estimated 15,000 to 20,000 Americans have been diagnosed with this disease. In the UK, it is estimated to have about 1 case for every 100,000 people. Globally, males are affected more frequently than females. In the United States, more females are affected than males.

In an epidemiologic study, 56 percent of patients with Behçet's disease developed ocular involvement at a mean age of 30. Ocular involvement was the first manifestation of Behçet's disease in 8.6 percent of patients. Ocular Behçet's disease with involvement of the optic nerve is rarely reported. Among patients with ocular Behçet's disease funduscopic findings of optic atrophy, and optic disc paleness have been identified with a frequency of 17.9 percent and 7.4 percent, respectively. Other fundoscopic findings include vascular sheathing (23.7%), retinal hemorrhage (9%), macular edema (11.3%), branch retinal vein occlusion (5.8%), and retinal edema (6.6%). However, optic atrophy was the most significant cause of visual impairment identified in 54 percent of patients with ocular Behçet's disease and permanent visual impairment.

The prevalence of this disease increases from North to South. It follows a more severe course in patients with an early age of onset particularly in patients with eye and gastrointestinal involvement.

This is a partial list of human eye diseases and disorders.

The World Health Organization publishes a classification of known diseases and injuries, the International Statistical Classification of Diseases and Related Health Problems, or ICD-10. This list uses that classification.

While risk factors vary with age and gender, most of the common chronic diseases in the US are caused by dietary, lifestyle and metabolic risk factors that are also responsible for the resulting mortality. Therefore, these conditions might be prevented by behavioral changes, such as quitting smoking, adopting a healthy diet, and increasing physical activity. Social determinants are important risk factors for chronic diseases. Social factors, e.g., socioeconomic status, education level, and race/ethnicity, are a major cause for the disparities observed in the care of chronic disease. Lack of access and delay in receiving care result in worse outcomes for patients from minorities and underserved populations. Those barriers to medical care complicate patients monitoring and continuity in treatment.

In the US, Minorities and low-income populations are less likely to access and receive preventive services necessary to detect conditions at an early stage.

The majority of US health care and economic costs associated with medical conditions are for the costs of chronic diseases and conditions and associated health risk behaviors. Eighty-four percent of all health care spending in 2006 was for the 50% of the population who have one or more chronic medical conditions (CDC, 2014).

Adult-onset Still's Disease is rare and has been described all over the world. The number of new cases per year is estimated to be 1.6 per 1,000,000 population. The number of people currently affected is estimated at 1.5 cases per 100,000-1,000,000 population. Onset is most common in two age ranges, between ages 15–25 and between ages of 36–46 years.

This disease is more common in women and an association with the gene FLT4 has been described. FLT4 codes for VEGFR-3, which is implicated in development of the lymphatic system.

Milroy's disease is also known as primary or hereditary lymphedema type 1A or early onset lymphedema.

It is a very rare disease with only about 200 cases reported in the medical literature. Milroy's disease is an autosomal dominant condition caused by a mutation in the FLT4 gene which encodes of the vascular endothelial growth factor receptor 3 (VEGFR-3) gene located on the long arm (q) on chromosome 5 (5q35.3).

In contrast to Milroy's disease (early onset lymphedema type 1A,) which typically has its onset of swelling and edema at birth or during early infancy, hereditary lymphedema type II, known as Meige disease, has its onset around the time of puberty. Meige disease is also an autosomal dominant disease. It has been linked to a mutations in the ‘forkhead’ family transcription factor (FOXC2) gene located on the long arm of chromosome 16 (16q24.3). About 2000 cases have been identified. A third type of hereditary lymphedema, that has an onset after the age of 35 is known as lymph-edema tarda.

The disease is regarded as extremely rare, with an incidence (new number of cases per year) of one case per million people. The patients are predominantly male (86% in a survey of American patients), although in some countries the rate of women receiving a diagnosis of Whipple's disease has increased in recent years. It occurs predominantly in those of Caucasian ethnicity, suggesting a genetic predisposition in that population.

"T. whipplei" appears to be an environmental organism that is commonly present in the gasterointestinal tract but remains asymptomatic. Several lines of evidence suggest that some defect—inherited or acquired—in immunity is required for it to become pathogenic. The possible immunological defect may be specific for "T. whipplei", since the disease is not associated with a substantially increased risk of other infections.

The disease is usually diagnosed in middle age (median 49 years). Studies from Germany have shown that age at diagnosis has been rising since the 1960s.

There are no currently known causes of this disease. There are studies currently proposing several theories of the causes which include inflammation of the adipose tissue, nervous system malfunction and endocrine malfunction. None of the theories that are currently proposed have been found viable. Since little is known about Dercum's disease, there are currently no known modes of prevention. Some hypotheses state that maintaining a healthy weight and diet can help prevent Dercum's although it has not been proven.

Dercum's disease can affect people of any gender and of any age. The majority of cases are linked to women between the ages of 45 and 60, who are overweight and postmenopausal. Due to the difficulty of diagnosis of this disease, many cases are underreported or misdiagnosed and it is difficult to understand what part of the population is affected by it the most.

Morbidity and mortality range from both extremes as the significance correlate with the underlying systemic disease.

The following are not classified as diseases of the eye and adnexa (H00-H59) by the World Health Organization:

- (B36.1) Keratomycosis — fungal infection of the cornea

- (E50.6-E50.7) Xerophthalmia — dry eyes, caused by vitamin A deficiency

- (Q13.1) Aniridia — a rare congenital eye condition leading to underdevelopment or even absence of the iris of the eye

A growing body of evidence supports that prevention is effective in reducing the effect of chronic conditions; in particular, early detection results in less severe outcomes. Clinical preventive services include screening for the existence of the disease or predisposition to its development, counseling and immunizations against infectious agents. Despite their effectiveness, the utilization of preventive services is typically lower than for regular medical services. In contrast to their apparent cost in time and money, the benefits of preventive services are not directly perceived by patient because their effects are on the long term or might be greater for society as a whole than at the individual level.

Therefore, public health programs are important in educating the public, and promoting healthy lifestyles and awareness about chronic diseases. While those programs can benefit from funding at different levels (state, federal, private) their implementation is mostly in charge of local agencies and community-based organizations.

Studies have shown that public health programs are effective in reducing mortality rates associated to cardiovascular disease, diabetes and cancer, but the results are somewhat heterogeneous depending on the type of condition and the type of programs involved. For example, results from different approaches in cancer prevention and screening depended highly on the type of cancer.

The rising number of patient with chronic diseases has renewed the interest in prevention and its potential role in helping control costs. In 2008, the Trust for America's Health produced a report that estimated investing $10 per person annually in community-based programs of proven effectiveness and promoting healthy lifestyle (increase in physical activity, healthier diet and preventing tobacco use) could save more than $16 billion annually within a period of just five years.

Urbach–Wiethe disease is very rare; there are fewer than 300 reported cases in medical literature. Although Urbach–Wiethe disease can be found worldwide, almost a quarter of reported diagnoses are in South Africa. Many of these are in patients of Dutch, German, and Khoisan ancestry. This high frequency is thought to be due to the founder effect. Due to its recessive genetic cause and the ability to be a carrier of the disease without symptoms, Urbach–Wiethe disease often runs in families. In some regions of South Africa, up to one in 12 individuals may be carriers of the disease. Most of the case studies involving Urbach–Wiethe disease patients involve only one to three cases and these cases are often in the same family. Due to its low incidence, it is difficult to find a large enough number of cases to adequately study the disease.

Adult-onset Still's disease is treated with anti-inflammatory drugs. Steroids such as prednisone are used to treat severe symptoms of Still's. Other commonly used medications include hydroxychloroquine, penicillamine, azathioprine, methotrexate, etanercept, anakinra, cyclophosphamide, adalimumab, rituximab, and infliximab.

Newer drugs target interleukin-1 (IL-1), particularly IL-1β. A randomized, multicenter trial reported better outcomes in a group of 12 patients treated with anakinra than in a group of 10 patients taking other disease-modifying antirheumatic drugs. Other anti-IL1β drugs are being developed, including canakinumab and rilonacept.

The condition "juvenile-onset Still's disease" is now usually grouped under juvenile rheumatoid arthritis. However, there is some evidence that the two conditions are closely related.

"T. whipplei" is one of the actinomycetes, and is a distant relative of "Mycobacterium avium-intracellulare", and "Mycobacterium paratuberculosis" explaining in part why Whipple's disease is similar to the diseases caused by these bacteria. The disease is common in farmers and those exposed to soil and animals, suggesting that the infection is acquired from these sources.

Individuals who are most susceptible to the disease are those with decreased ability to perform intracellular degradation of ingested pathogens or particles, particularly in the macrophages. Several studies indicate that defective T-lymphocyte (particularly TH1 population) function may be an important predisposing factor for the disease. In particular circulating cells that are CD11b (also known as integrin alpha) expressive, are reduced in susceptible individuals. CD11b has a vital role in activation of macrophages to destroy intracellularly ingested "T. whipplei" bacteria.

There seems to be beneficial responses to clindamycin therapy as the lesions regress. This leads to the hypothesis that microorganisms may be playing a role in the initial stages of Kyrle disease.

A family with Kyrle disease were examined which their skin lesions were benign. However, when three of the young adult members were closely examined, they had posterior subcapsular cataracts and two of those three developed multiple tiny yellow-brown anterior stromal corneal opacities. In order to determine if there is any correlation between Kyrle disease and the ocular observations, more cases of Kyrle disease are to be analyzed.

All in all, since Kyrle disease is relatively rare, more cases need to be studied and analyzed in order to understand the underlying pathogenesis and to improve the management of the disease.

Dercum's disease most commonly appears between the ages of 35 and 50 years of age. It is five to thirty times more common in women than in men. Originally, Dercum proposed that the condition mainly affects postmenopausal women. However, a 2007 survey has revealed that 85.7 percent of the included patients developed Dercum's disease before menopause. The prevalence of Dercum's disease has not yet been exactly established.

While the exact cause is unknown, Crohn's disease seems to be due to a combination of environmental factors and genetic predisposition. Crohn's is the first genetically complex disease in which the relationship between genetic risk factors and the immune system is understood in considerable detail. Each individual risk mutation makes a small contribution to the overall risk of Crohn's (approximately 1:200). The genetic data, and direct assessment of immunity, indicates a malfunction in the innate immune system. In this view, the chronic inflammation of Crohn's is caused when the adaptive immune system tries to compensate for a deficient innate immune system.

The increased incidence of Crohn's in the industrialized world indicates an environmental component. Crohn's is associated with an increased intake of animal protein, milk protein and an increased ratio of omega-6 to omega-3 polyunsaturated fatty acids.

Those who consume vegetable proteins appear to have a lower incidence of Crohn's disease. Consumption of fish protein has no association.

Smoking increases the risk of the return of active disease (flares). The introduction of hormonal contraception in the United States in the 1960s is associated with a dramatic increase in incidence, and one hypothesis is that these drugs work on the digestive system in ways similar to smoking. Isotretinoin is associated with Crohn's. Although stress is sometimes claimed to exacerbate Crohn's disease, there is no concrete evidence to support such claim. Dietary microparticles, such as those found in toothpaste, have been studied as they produce effects on immunity, but they were not consumed in greater amounts in patients with Crohn's.

Milroy's disease (MD) is a familial disease characterized by lymphedema, commonly in the legs, caused by congenital abnormalities in the lymphatic system. Disruption of the normal drainage of lymph leads to fluid accumulation and hypertrophy of soft tissues. It is also known as Milroy disease, Nonne-Milroy-Meige syndrome and hereditary lymphedema.

It was named by Sir William Osler for William Milroy, a Canadian physician, who described a case in 1892, though it was first described by Rudolf Virchow in 1863.

Life expectancy with Fabry disease for males was 58.2 years, compared with 74.7 years in the general population, and for females 75.4 years compared with 80.0 years in the general population, according to registry data from 2001 to 2008. The most common cause of death was cardiovascular disease, and most of those had received kidney replacements.

The twins require the use of wheelchairs for mobility and are unable to speak without the assistance of electronic speaking aids. They experience persistent and painful muscle spasms which are worsened by emotional distress. They are currently living with their parents, with the assistance of hospice workers. Doctors continue to administer tests to the twins in search of a treatment.

The disease appears to be progressive in nature. The Fields twins started having problems when they were four years old. By the time they had reached the age of nine, they were having difficulty walking and needed frames to assist them with walking. Their muscles have been gradually deteriorating over time. The disease affects the twins' nerves, causing them to make involuntary muscle movements such as trembling in the hands.

The extent of the disease is still unknown as the two women are only 21. However, the disease has had no apparent effect on their brains or personalities. Doctors do not know if the disease is fatal and, if so, what the life expectancy of one with this disease is. If the cause of the disease is genetic, there is a chance that the twins could pass it on to their future children.

Extramammary Paget's disease is usually seen in isolation and is associated with an underlying invasive malignancy about 12% of the time. It is associated with an underlying adnexal malignancy about 24% of the time. Paget's disease of the breast is almost always associated with an underlying invasive malignancy, i.e. breast cancer (e.g. mammary ductal carcinoma).