Feline infectious peritonitis (FIP) is a fatal incurable disease that affects cats. It is believed by some to be caused by Feline Infectious Peritonitis Virus (FIPV), which is a mutation of Feline Enteric Coronavirus (FECV) - (Feline Coronavirus FeCoV). Although there appears to be a connection between FIP and feline coronavirus, no clear cause and effect has yet been proven. Experts do not always agree on the specifics of FIP. However, the most common theory is that the normally benign FECV mutates into FIPV. The mutated virus has the ability to invade and grow in certain white blood cells, namely macrophages. The immune system's response causes an intense inflammatory reaction in the containing tissues. This disease is generally fatal. However, its incidence rate is roughly 1 in 5000 for households with one or two cats. A nasally administered vaccine for FIP is available, but controversial and isn't proven to be highly effective.