Gastroenterology Consultants,SC: Diseases of the GI tract ULCERATIVE COLITIS [<<back | list all>>] Ulcerative colitis is a chronic, recurring disease of the large bowel. The large bowel dehydrates the liquid stool that enters it and stores the formed stool until a bowel movement occurs. When ulcerative colitis affects the colon, inflammation and ulcers form in the lining of the colon. The cause is unknown, but some experts believe there may be a defect in the immune system that actually injure the colon. Others say that an unidentified microorganism or germ is responsible. The disorder typically begins gradually, with crampy abdominal pain and diarrhea. Loss of appetite and weight loss occur.