Crohn's disease causes inflammation of the full thickness of the bowel wall and may involve any part of the digestive tract from the mouth to the anus. Ulcerative colitis is inflammation confined to the inner lining of the large bowel (colon and rectum). These diseases are commonly called inflammatory bowel disease (IBD). IBD tends to develop between 15 and 30 years of age but can start at any age. There is a marked increase in the number of children under the age of nine years now developing Crohn's disease.