http://digestive.niddk.nih.gov/ddiseases/pubs/colitis/
National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health
In ulcerative colitis, the inner lining of the large intestine (colon or bowel) and rectum becomes inflamed. The inflammation usually begins in the rectum and lower (sigmoid) intestine and spreads upward to the entire colon. Ulcerative colitis rarely affects the small intestine except for the lower section, the ileum.