Appendicitis is an inflammation of the vermiform appendix, a finger-like appendage to the cecum, the first part of the large intestine. The inflammation results from a bacterial infection that causes the appendix to swell and fill with pus. An early symptom of appendicitis is intermittent pain in the navel region. This becomes more severe and, usually within hours, localizes to the lower, right side of the abdomen. The abdominal muscles tighten, and the person loses his or her appetite and becomes nauseated. A slight fever is usual, as is constipation. (The inflammation, however, may on occasion trigger diarrhea.) The lower abdomen is tender; touching increases the pain. Appendicitis is a common condition (1 to 2 cases per 1000 people annually). Males between ages 10 and 30 are most commonly affected.

The diagnosis of appendicitis can often be made based on the patient's description of the pain, coupled with the expected findings on examination of the abdomen.  In many cases, a CAT scan of the abdomen will help to confirm appendicitis as the source of the symptoms.