According to wiki:
[B]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/FeCoV. 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.
There are two main forms of FIP: effusive (wet) and non-effusive (dry). While both types are fatal, the effusive form is more common (60-70% of all cases are wet) and progresses more rapidly than the non-effusive form.
The hallmark clinical sign of effusive FIP is the accumulation of fluid within the abdomen or chest, which can cause breathing difficulties. Other symptoms include lack of appetite, fever, weight loss, jaundice, and diarrhea.
Dry FIP will also present with lack of appetite, fever, jaundice, diarrhea, and weight loss, but there will not be an accumulation of fluid. Although typically a cat with dry FIP will show ocular or neurological signs. For example it may become hard to walk or stand up, the cat may become paralyzed over time. There could also be a loss of sight.[/B]