There are about 1.4 million Americans who have IBD and 30,000 new cases are diagnosed each year. Young people between the ages of 15 and 35 are most likely to get IBD, but anyone can. The disease is most common among people of eastern European backgrounds, including Jews of European descent. In recent years, an increasing number of cases have been reported among African American populations.

Ulcerative colitis affects only the colon. It usually involves the rectum and can affect areas up into the colon. If it affects the whole colon, it is called pancolitis.

Crohn's disease can involve any part of the gastrointestinal tract, from the mouth to the anus.

Ulcerative colitis involves only the lining of the bowel (not the entire thickness of the bowel wall).

Crohn's disease affects the entire thickness of the intestinal wall.

Patients with ulcerative colitis and those with Crohn's disease that involves the colon have a greater risk of getting colon cancer, and this risk increases over time.

Although the exact cause of IBD is not known, the following are thought to play a role in both ulcerative colitis and Crohn's:

• Genetic tendency: Certain genes that patients were born with make getting IBD more likely.
• Environmental trigger: Exposure to certain medications, infections or toxins can cause IBD.
• Immune system: Inappropriate reactions by the immune system can cause IBD. For example, it interprets good bacteria as something harmful and attacks it, which causes inflammation.
• Bacteria that are normally in the intestine: The good and bad bacteria that belong in the intestine are not balanced.