Feline infectious peritonitis. FIP-infected kidney showing inflammatory response. Feline infectious peritonitis (FIP) is a fatal, incurable disease that affects cats. It is caused by Feline Infectious Peritonitis Virus (FIPV), which is a mutation of Feline Enteric Coronavirus (FECV) – (Feline coronavirus FCoV). Experts do not agree on the specifics of genetic changes that produce the FIPV.