Like Crohn's disease, ulcerative colitis causes inflammation and ulcers in your intestine. But unlike Crohn's, which can affect the colon in various sections, ulcerative colitis usually affects one continuous section of the inner lining of the colon beginning with the rectum.

No one is quite sure what triggers ulcerative colitis, but there's a consensus as to what doesn't. Researchers no longer believe that stress is the main culprit, although stress can often aggravate symptoms. Instead, current thinking focuses on the following possibilities:

- Immune system. Some scientists think a virus or bacterium may trigger ulcerative colitis. The digestive tract becomes inflamed when your immune system tries to fight off the invading microorganism (pathogen). It's also possible that inflammation may stem from an autoimmune reaction in which your body mounts an immune response even though no pathogen is present.

- Heredity. Because you're more likely to develop ulcerative colitis if you have a parent or sibling with the disease, scientists suspect that genetic makeup may play a contributing role.