Appendicitis: This is often the first consideration in cases of peritonitic abdomen, especially if the pain is localized to the lower right quadrant. The inflammation of the appendix can lead to perforation, causing peritonitis.

Diverticulitis: Inflammation of a diverticulum in the colon can rupture, leading to peritonitis. This condition is more common in older adults.
Gastrointestinal Perforation: A hole in the wall of the stomach or intestine can allow bacteria to leak into the abdominal cavity, causing peritonitis. This could be due to ulcers, trauma, or other conditions.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones, can lead to perforation and peritonitis if not treated promptly.

Ectopic Pregnancy: Although less common, a ruptured ectopic pregnancy can cause severe abdominal pain and peritonitis. It's crucial to consider this in women of childbearing age.
Ovarian Torsion: Twisting of an ovary can cut off its blood supply, leading to severe pain and potentially peritonitis if the ovary ruptures.
Mesenteric Ischemia: Reduced blood flow to the intestines can cause tissue death, leading to peritonitis. This condition requires immediate medical attention.

Primary Peritonitis: Spontaneous bacterial peritonitis without an obvious source of infection, more common in patients with cirrhosis or ascites.
Tuberculous Peritonitis: A form of tuberculosis that affects the peritoneum, more common in areas where TB is prevalent.
Pneumococcal Peritonitis: Caused by Streptococcus pneumoniae, this is a rare cause of peritonitis, often associated with pneumonia or other infections.