Acute appendicitis is inflammation of the appendix, the narrow, finger-shaped organ that branches off the first part of the large intestine on the right side of the abdomen. Although the appendix is a vestigial organ with no known function, it can become diseased. In fact, acute appendicitis is the most common reason for abdominal surgery in the world. If it is not treated promptly, there is the chance that the inflamed appendix will burst, spilling fecal material into the abdominal cavity. The usual result is a potentially life-threatening infection (peritonitis), but the infection may become sealed off and form an abscess. Appendicitis is uncommon among older people, and symptoms are generally mild, so that diagnosis of the acute episode is often not made. Members of this age group are thus at greater risk for rupture with peritonitis or abscess formation.