The exact cause of ulcerative colitis is not known. It is, however, considered to be an autoimmune disease.

Normally the body’s immune system works to defend the body against infection and invading microbes. In autoimmune disorders, this goes awry and the immune system attacks body’s own cells.

There are billions of harmless bacteria present in the gut. In inflammatory bowel diseases like Crohn’s disease and Ulcerative colitis, the immune system targets these harmless bacteria inside the colon and in turn attacks the tissues of the colon, causing it to become inflamed.

The cause why the immune system behaves in this manner is unclear.

Some of the hypotheses regarding cause of ulcerative colitis include:-

The overactive immune system

After the immune system is done fighting the invading viruses and bacteria that triggers it, it does not switch off. Continued vigilant and active immune system leads to inflammation.

Some research shows that the immune system is actually not triggered by harmful invading bacteria but by billions of harmless or friendly bacteria present in the gut. This is a manifestation of the autoimmune hypothesis behind causation of ulcerative colitis.

Genes

There are population studies that reveal that at least 1 in 6 persons with ulcerative colitis have a blood relative who has this condition.
 

The genetic hypothesis is also strengthened by the fact that some communities are more at risk of this condition than others. It affects white people of European descent especially those from Ashkenazi Jewish communities and black people more than others. It is rare among people of Asian descent.

Researchers have identified several genes that seem to predict if a person would develop ulcerative colitis but the exact mechanism is not yet known.

Environmental factors

It is seen that ulcerative colitis is more common among urban areas in northern parts of Western Europe and America. There have been studies showing an association of ulcerative colitis with several environmental factors including air pollution, diet and hygiene.

It is believed that typical Western diet is high in carbohydrates and fats. This is markedly different from the Asian diet that is lower in carbohydrates and fats. Generations of Westernized diet could be the key to causation of this condition. Studies have shown that ulcerative colitis as well as Crohn’s disease is more prevalent in white collar compared with blue-collar occupations.

In addition, children brought up in increasingly germ-free environments fail to be exposed to necessary microbes that help in strengthening the immune system. This is called the hygiene hypothesis and suggests why those living in developing and poorer countries with lower hygiene standards are relatively less likely to develop ulcerative colitis. This along with other hypotheses like oral contraceptive use, atypical mycobacterial infections etc. have not been proven to raise the risk of ulcerative colitis specifically.

Smoking is another notable environmental factor. Ulcerative colitis is more prevalent among ex-smokers and non-smokers while Crohn’s disease is more prevalent among smokers.