FIP is actually a MUTATION of the very common Feline Coronavirus (FECV)
Almost every domestic cat will be exposed to the Feline Coronavirus in his or her lifetime and about 90% of cats who are exposed will either show ZERO symptoms or just have what amounts to a stomach bug and go on to make a full recovery
It’s the unfortunate 10% of the population who will experience the mutation of the virus into FIP
Because FIP is a result of a mutation, it is not currently believed to be contagious from cat to cat