Crohn's disease. Light micrograph of a section through the lining of the colon (large intestine) of a patient with Crohn's disease, showing a fissured ulcer (central indentation). This is a chronic inflammatory disorder of the intestines that causes ulceration and thickening of the lining. The cause of the disease is not clear, although it may be due to either an overactive or underactive immune system. There is no cure, although anti-inflammatory drugs and surgical removal of badly affected parts of the gut are often used.