Feline infectious peritonitis (FIP) is a viral disease of cats caused by certain strains of a virus called the feline coronavirus. Most strains of feline coronavirus are relatively harmless and cause mild diarrhea, but some strains can mutate and cause FIP in some cats. Cats that are housed in high-density facilities such as shelters, catteries, or multi-cat households appear to be more susceptible to the development of FIP, as are purebred cats, male cats, and geriatric cats, for reasons that remain unclear. Genetic factors are also thought to contribute to the development of FIP.

FIP can present in two major forms: an effusive, or “wet” form and a noneffusive, or “dry” form. Regardless of which form they ultimately progress to develop, cats infected with FIP usually first develop nonspecific signs of disease such as loss of appetite, weight loss, depression, and fever. The signs of the effusive form of FIP usually develop and progress relatively rapidly and include the accumulation of fluid in body cavities, including the abdomen and the thorax (chest cavity) . Affected cats may develop a pot-bellied appearance due to fluid accumulation in the abdomen, and if the fluid accumulation is excessive, it may become difficult for a cat to breathe normally. The dry form of FIP causes infection and inflammatory lesions around the blood vessels in a cat’s body, which can affect the brain, liver, kidneys, lungs, and skin. This form often causes the cat to have seizures and move in an abnormal or uncoordinated way. In some cases, cats will also have excessive thirst and urination, vomiting, weight loss, and jaundice.

The factors determining why one cat becomes infected with FIP while another cat does not are not fully understood[[2]](http...