A diverticulum is a bulge of a round mass in the colon. It occurs when the inner lining of the colon weakens and forms one or more pouches. When these pouches become inflamed or infected, it is called diverticulitis.

Diverticulitis is a disease with one or more inflamed diverticula. Inflammation spreads to surrounding fatty tissue, sometimes perforation of the diverticulum leads to abscess formation or peritonitis. Rarely, a colonic fistula may form with the bladder, fallopian tubes, uterus, and vagina.

Appendicitis and diverticulitis are two diseases with similar clinical manifestations. It is more difficult to accurately diagnose diverticulitis before surgery than for appendicitis, so most diagnoses are made during surgery.