Crohn's disease of the colon. Section of ascending colon affected by Crohn's disease. Crohn's disease causes inflammation, thickening, and ulceration of the intestinal tract. It often occurs in the ileum of the small intestine and in the colon (large intestine) but can affect any part of the intestinal tract, which includes anywhere from the mouth to the anus. Its cause is unknown and there is no cure. Symptoms include abdominal pain, diarrhoea and the malabsorption of food, leading to weight loss.