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.