Feline Infectious Peritonitis is an unusual disease. Although infectious, it can seem to appear out of nowhere. When exposed, some cats may become "infected", while others don't. It is difficult to diagnose, especially in what's referred to as the "dry form". Once a cat is infected, it is considered to be always fatal. There's no effective treatment. Currently, there's no 100% diagnostic test.
Don't confuse Feline infectious Peritonitis, referred to as FIP, with FIV, a totally unrelated virus, Feline Immunodifficiency virus.
FIP is caused by a coronavirus. Diarrhea caused by feline coronavirus is extremely common, to the point that virtually all cats become exposed to it. Most cats develop only mild disease and recover uneventfully. This virus, however, mutates frequently, and instead of thinking about this as a group of identical viruses, you should think about this as a cloud of viruses, each slightly different. Unfortunately, some of these mutations become the deadly FIP viruses.
Picture one person, we'll call him Max. Imagine a whole planet of people that looked exactly like Max. That's how we often picture a population of viruses. Of course, the humans don't all look identical. Everyone looks different, yet, they're all still human. Now imagine that some humans have a mutation that gives them a bent pinky finger, and this mutation makes them deadly when they contact other humans. These deadly people would still be considered human, and looking at the planet from space, you wouldn't be able to pick out the bent-pinky people. That's a stylized version of how FIP relates to enteric (intestinal) coronavirus.
Now, similarly, we can say that normal people can give birth to bent-pinky poeople. So the bent-pinky people can show up where they're not expected. Bent-pinky people have a greater chance of producing bent-pinky people, and that's how you get outbreaks that spred through a population.
Now, here's another unusual aspect of this disease. The virus itself does not cause the illness. The illness is caused by an abnormal antibody response to the virus. The antibody response is ineffective in fighting the virus, and the antibody response itself causes the deadly disease. Some exposed individuals produce this abnormal response, while others do not, and therefore, don't get the disease.
Because the disease is difficult to diagnose, veterinarians are unsure how often they're actually seeing it. Twenty years ago, about half of the cats with FIP were also infected with feline leukemia. The feline leukemia vaccines have reduced the incidence of feline leukemia. The incidence of FIP has also decreased, and feline leukemia is less of a factor in FIP infection than it was previously. But just like 20 years ago, there is no effective treatment for FIP, and no accurate tests to definitely diagnose it.
A vaccine for FIP has been available for some time. The effectiveness and usefulness of the vaccine has been questioned. Ask your veterinarian it the vaccine is recommended for your Cat(s). For more information on the FIP vaccine, click on the link below.