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 a disease. Normally the disease/virus relationship is simple, but this is not the case with FIP. FIP may be caused by many things, perhaps an isolated FIP virus (FIPV), perhaps a mutation of FECV, or perhaps there are multiples viruses which can all lead the the same disease complex known as FIP. There is little question, however, that the most common cause of FIP is via FECV.
For the most part, FECV is limited largely to the intestines and is dealt with quite well by the cat's immune system. However, as recent studies seem to indicate, FECV can mutate into FIP and, if the cat's immune system is not operating properly, this mutant FECV stops being just an infection of the intestine and becomes the more systemic infection we call FIP.
Thus, wherever you have FECV you could have FIP! Some cats never get FIP, but can continue to shed the FECV virus (now thought to be spread via the feces). The good news, however, is that since it seems that the dry form is becoming more prevalent, that cats are gradually becoming more able to resist FIP infection in general.