A gastrointestinal system disease that involves inflammation of the peritoneum resulting from perforation of the gastrointestinal tract, which produces immediate chemical inflammation followed shortly by infection from intestinal organisms. Peritonitis can also result from appendicitis, diverticulitis, strangulating intestinal obstruction, pancreatitis, pelvic inflammatory disease, mesenteric ischemia, intraperitoneal blood, barium, or peritoneo-systemic shunts, drains, and dialysis catheters in the peritoneal cavity. The symptoms include abdominal pain and tenderness, fever, fluid in the abdomen, nausea, vomiting and low urine output.