Peritonitis is a frequently life-threatening infection (or an inflammation) of the peritoneum, which is a two-layered membrane covering both the surfaces of the organs that lie in the abdominal cavity and the inner surface of the abdominal cavity itself. 
A major cause of bacterial peritonitis is internal perforation of the gastrointestinal tract, contaminating the abdominal cavity with bacteria from gastric contents. This may result as a complication of an intestinal foreign body or a ruptured appendix.