The most common forms of chronic inflammatory bowel disease, Crohn's disease and ulcerative colitis, are related, but show some important differences, primarily in the site of inflammation and the way it behaves. In ulcerative colitis, the inflammation is limited to the innermost layer of the intestinal wall, the mucous membrane. Crohn's disease can lead to narrowing of the bowel or perforation with formation of unnatural connections called fistulas, because the inflammation often penetrates deeper into the bowel wall. The course of the disease varies greatly from patient to patient and not everyone experiences the same effect from the available treatments.