Peritonitis occurs when the thin layer of tissue lining the tummy (peritoneum) becomes infected with bacteria or fungi.

The infection can either develop directly in the peritoneum or spread from another part of the body.

Infection of other parts of the body

Most often, peritonitis is caused by an infection that spreads to the peritoneum from another part of the body. This is known as secondary peritonitis.

Common causes of secondary peritonitis include:

• a split stomach ulcer
• a burst appendix
• inflammation of the pancreas (acute pancreatitis)
• severe trauma to the abdomen, such as a knife or gunshot wound
• digestive disorders, such as Crohn's disease or diverticulitis

Both Crohn's disease and diverticulitis can cause the colon to become inflamed. If the inflammation is particularly severe, the colon can split and leak the contents of the bowel into the peritoneum, contaminating it with bacteria.

Direct infection

Peritonitis can occur when the peritoneum becomes directly infected, although this is rare. This is known as primary or spontaneous peritonitis.

In most cases, primary peritonitis is caused by scarring of the liver or peritoneal dialysis.

Cirrhosis

Scarring of the liver, known as cirrhosis, can occur after liver damage. The most common causes of cirrhosis in the UK are alcohol misuse, a hepatitis C infection, or obesity.

Cirrhosis can lead to a build-up of fluid inside the abdomen (ascites). This fluid is particularly vulnerable to infection, and provides an environment where bacteria or fungi can grow and spread.

This build-up of fluid affects around half of people with cirrhosis, usually many years after cirrhosis was diagnosed. About 20% of people with cirrhosis who develop ascites will be affected by peritonitis.

Peritoneal dialysis

People having peritoneal dialysis for kidney failure are at risk of developing peritonitis. Dialysis is a medical treatment that replicates the main functions of the kidneys and removes waste products from the body.

Peritoneal dialysis uses the peritoneum to replicate the main function of the kidneys, which is to filter waste products out of the blood. A small tube called a catheter is implanted into the peritoneum, which removes waste products.

Although rare, peritonitis can develop if the equipment becomes contaminated by bacteria or fungi.