Is it possible to have chronic untreated peritonitis, or is there no such thing because untreated peritonitis is fatal?

Peritonitis: Peritonitis is generally not fatal. Yes it can be chronic. Causes include liver failure,bacterial & viral infection, chemical, drug induced etc.....so it depends on the type what the treatment is. Treatment may be in the form of an antibiotics..or if there's fluid in the peritoneum..it may require drainage. The root cause needs to be identified. A gastroenterologist is the place to start.