One popular theory is that a combination of genetic, environmental and infectious factors may cause a fault in the immune system, causing inflammation of the bowel.
Studies have shown that you are slightly more likely to develop Crohn's disease or colitis if you have a close relative (mother, father, sister or brother) with the disease. Certain ethnic groups are also more likely to develop these conditions.
This suggests that certain genes may play a role in causing these conditions to develop.
Researchers have identified several genes that are more common in people with Crohn's disease or colitis, but they have not been able to show that any of these genes actually causes these conditions.
Crohn's disease and colitis are more common in people living in Australia, Western Europe and America than they are in developing countries in Asia and Africa. This has led some researchers to suggest that the modern lifestyle may play some role in causing Crohn's disease and colitis in susceptible people. There is no hard scientific evidence to support this theory.
Some scientists think that a previous bacterial infection can trigger an abnormal immune response in some people, causing Crohn's disease or colitis to develop. The role of infection as a trigger for these conditions is under investigation.
Sources: Digestive Health Foundation (Publication - GESA, DHF. Information about Inflammatory Bowel Disease (IBD) Crohn's disease and ulcerative colitis. (2011)), NHS Choices, UK (Crohn's disease - Causes, Ulcerative colitis - Causes)
Last reviewed: September 2015