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.