The idiopathic inflammatory bowel diseases (IBD)—Crohn's disease (CD) and ulcerative colitis (UC)—are chronic, frequently disabling diseases of the gastrointestinal tract. CD and UC have an estimated combined prevalence of 0.2% to 0.3% in the United States, and are two to eight times more prevalent in Jewish Americans of Central or Eastern European descent (Ashkenazi Jews) as compared to non-Jewish Americans. CD may involve any part of the gastrointestinal tract, but most often the colon and terminal ileum. Bowel inflammation is discontinuous, transmural, and may contain granulomas. CD is also associated with bowel stenoses and intestinal and perianal fistulas. UC, by contrast, involves continuous, non-granulomatous inflammation of the colon and rectum, limited to the mucosal layers. Fistulas are not observed. Colonic disease that cannot be clearly identified as CD or UC is designated "indeterminate colitis".