There are two major forms of peritonitis – primary and secondary. Primary peritonitis, in which the source of infection is not known, is often caused by the spread of bacteria in the blood or lymph nodes.

Primary peritonitis is also known as primary bacterial peritonitis (PBP) and spontaneous bacterial peritonitis (SBP). This type of peritonitis is often seen in patients with cirrhosis of the liver, especially those with a history of alcoholism. Primary peritonitis is also associated with other conditions, such as:

• Hepatitis, or inflammation of the liver
  
  
• Metastatic cancers, or cancers that have spread throughout the body
  
  
• Congestive heart failure, in which the heart fails to pump enough blood to meet the body's needs
  
  
• Systemic lupus erythematosus, an inflammatory, autoimmune disease that affects many parts of the body

Primary peritonitis may also occur in patients with no history of underlying disease. Patients who have ascites (fluid buildup in the abdomen) can develop PBP because this fluid provides a good environment for bacteria to grow.

Secondary peritonitis develops when the peritoneum becomes inflamed due to the spillage of gastric contents, which may include digestive enzymes or bacteria. The digestive system contains both naturally occurring bacteria and enzymes that assist in the breakdown of nutrients. These substances are necessary for proper digestion but can cause harm if they escape through a hole (perforation) in one of the abdominal organs.

Because the peritoneum is a conduit for blood and lymph vessels, it is possible for bacteria or toxins to spread to other parts of the body.

A third type of peritonitis may occur in patients undergoing a type of dialysis treatment called continuous ambulatory peritoneal dialysis (CAPD), which is used in patients with kidney failure. During this form of dialysis, fluid  is exchanged through a catheter inserted into the peritoneum to filter waste.