Peritonitis is an inflammation of the abdominal cavity and its lining (peritoneum) that typically also involves infection. The peritoneum is a two-layered membrane that covers the abdominal cavity and lines most of the abdominal organs (e.g., stomach, small and large intestines, appendix, gallbladder).

This membrane helps keep the abdominal organs moist and connects them to blood and lymph vessels. Peritonitis usually develops when an abdominal organ develops a hole (perforation), causing bacteria or digestive enzymes that naturally occur in the gastrointestinal tract to leak into the abdominal cavity. The result is infection and/or irritation.

Peritonitis can also occur if bacteria outside the body infiltrate the peritoneum (e.g., a stomach wound caused by trauma, surgery).

Peritonitis can be primary, in which the source of contamination is unknown, or secondary, in which the source has been identified. A third type of peritonitis can develop in patients undergoing certain types of dialysis (the artificial removal of bodily wastes after kidney failure).

Most cases of peritonitis are secondary. They often involve the rupture of an organ, such as the appendix or gallbladder, resulting in spillage of the organ’s contents into the abdominal cavity. The risk of peritonitis is a primary reason why appendicitis is considered to be a dangerous condition requiring prompt medical attention.