Ulcerative colitis is thought to be an autoimmune condition. This means the immune system – the body's defence against infection – goes wrong and attacks healthy tissue. The most popular theory is that the immune system mistakes harmless bacteria inside the colon for a threat and attacks the tissues of the colon, causing it to become inflamed. Exactly what causes the immune system to behave in this way is unclear. Most experts think it's a combination of genetic and environmental factors.