Peritonitis is inflammation of the abdominal cavity. The infection can originate with a foreign body lodged in the abdomen which creates an infection, infections from other parts of the abdomen such as fallopian tubes or appendix, or outside causes such as surgical trauma. Sepsis indicates an infection causing systemic symptoms and usually occurs when bacteria from local reactions enter the bloodstream.

One reason the terms are often confused is peritonitis is sometimes called abdominal sepsis. Symptoms of sepsis are fever, increased breathing and heart rate and infection present in another part of the body, such as a urinary tract infection or an infected tooth.

Peritonitis symptoms include abdominal pain, may involve vomiting or diarrhea, dehydration and increased heart rate (so some overlap exists). Peritonitis, a localized infection, may proceed to sepsis. Both conditions may be difficult to diagnose. Peritonitis may be more difficult because the medical team frequently needs to collect a sample of the fluid from the abdominal cavity, whereas sepsis typically requires only a blood draw. Both conditions can be fatal if left untreated.