Ulcerative colitis primarily affects young adults, but it can occur at any age from five to eighty years and women tend to be more commonly affected than men. It is a worldwide disorder with high-incidence areas that include United Kingdom, the United States, northern Europe and Australia. Low-incidence areas include Asia, Japan, and South America.

The causes of ulcerative colitis remain unknown. The major theories include infection, allergy to food component, genetics, environmental factors, and immune response to bacteria or other antigens.

Infection : Although use of antibiotics has been shown to improve treatment of active ulcerative colitis, no specific bacteria or virus have been isolated despite many attempts; therefore ulcerative colitis is unlikely to be due to a simple infection. Organisms such as shigella and pathogenic E. Coli have been investigated as possible infectious agents but they have not been associated with the etiology of the disease.

Food Allergy : The majority of patients with ulcerative colitis are found to be sensitive to milk and this led to claims that food allerg ies , particularly to milk, were the cause of the disease. Although some studies suggest that about twenty percent of patients with ulcerative colitis can benefit from a milk-free diet, there is very little scientific evidence to prove dietary allergy as the etiology of ulcerative colitis.

Environmental Factors : Other environmental factors proposed as contributing factors in the etiology of ulcerative colitis are oral contraceptives and smoking. The use of birth control pills was found to be more common in women with ulcerative colitis compared to women without the disease. Many studies have shown that ulcerative colitis is more common in nonsmokers than in smokers. The risk of developing ulcerative colitis is shown to be highest for previously heavy smokers, especially within the first two years of cessation. Smoking has shown to alter the function of the cells in the inner lining of the colon but the exact mechanism of the protective effect of smoking is unknown.

Genetics : There is some evidence that support the theory that ulcerative colitis may be due to genetic factors. The relatives of patients with ulcerative colitis are more likely to have the same disease compared to the general population and approximately ten to twenty percent of patients affected also have at least one other family member with the same disease. Numerous genes with a possible association with ulcerative colitis have been identified. Possible mechanisms proposed for genetic factors contributing to the cause of ulcerative colitis include metabolic defects and connective tissue disorders.

Autoimmune Disease : The current leading theory suggests that ulcerative colitis is an autoimmune disease, where the body's own immune system triggers an inflammatory response against an antigen in the intestinal wall. In this theory, the patient's immune system mounts an immune response against an antigen, either food particles or microbial organisms which the immune cells recognize as foreign objects, and this immune response triggers inflammatory effects. However, because of similarities between proteins on the cells of the inner lining of the colon and the antigens (dietary or microbial), the patient's own immune system is activated against the cells on the surface of the colon. Once the immune cells are activated, inflammatory mediators are released which not only cause tissue damage but also amplify the immune response and promote further inflammation in the inner lining of the colon.