Peritonitis is an inflammation of the peritoneum, the thin tissue lining the abdominal cavity and covering the abdominal organs. It can occur due to infection, injury, or underlying medical conditions. Common causes include appendicitis, perforated ulcers, or complications from diverticulitis. Symptoms often include severe abdominal pain, fever, nausea, and a rigid abdomen. If left untreated, peritonitis can lead to serious complications, such as sepsis, which is a life-threatening response to infection. Diagnosis typically involves a physical examination, imaging tests, and laboratory tests. Treatment usually requires antibiotics and may involve surgery to address the underlying cause.