Appendicitis is an inflammation of the appendix. The appendix is a small, fingerlike pouch located where the large and small intestines join. It has no known use. Inflammation is the body's reaction to irritation, injury, or infection. It causes swelling and pain. It is important to get treatment for appendicitis before the appendix ruptures. A rupture is a break or tear in the appendix. If an infected appendix ruptures, infection may move into the abdomen. This can cause peritonitis, which is a dangerous infection of the lining of the belly. Peritonitis can be fatal if it is not treated right away. Because of the risk of rupture, appendicitis is considered an emergency.