This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
FIP is not caused by a retrovirus but by a type of coronavirus. One of the reasons FIP is such a problem for vets is because there may be no way to differentiate an FIP virus from certain other viruses. Current thinking is that FIP is caused by a mutation of the Feline Enteric Coronavirus (FECV). FECV is very common, and an FECV infection can have symptoms ranging from none, to flu-like with or without diarrhea. These are most common in kittens, but can occur in cats of any age. If the immune system is not functioning properly, a mutant FECV can become a more systemic infection that we call FIP. All FIP tests appear to react the same way to every type of coronavirus. So, if your cat had FECV as a kitten, it may cause the same reaction in the current test as true FIP (more on the tests below).
For the purpose of this FAQ, however, I am going to continue referring to an "FIP Virus" or "FIPV." Just keep in mind that in fact, there may not be a difference between FIPV and FECV per se, just a difference in the way a cat's immune system responds.