Although the GI tract normally contains bacteria, the peritoneum is sterile. In peritonitis, however, bacteria invade the peritoneum. Generally, such infection results from inflammation and perforation of the GI tract, allowing bacterial invasion. Usually, this is a result of appendicitis, diverticulitis, peptic ulcer, ulcerative colitis, volvulus, strangulated obstruction, perforated or ruptured gallbladder, gangrenous gallbladder, abdominal neoplasm, or a penetrating wound.

Peritonitis may also result from chemical inflammation, as in the rupture of a fallopian tube or the bladder, perforation of a gastric ulcer, or release of pancreatic enzymes.

With chemical and bacterial inflammation, accumulated fluids containing protein and electrolytes make the transparent peritoneum opaque, red, inflamed, and edematous. Because the peritoneal cavity is so resistant to contamination, such infection is often localized as an abscess instead of disseminated as a generalized infection.

The key symptom of peritonitis is sudden, severe, and diffuse abdominal pain that tends to intensify and localize in the area of the underlying disorder.