FIP is a frustrating and ultimately fatal disease of cats, a type of corona virus that is widespread among the normal intestinal flora of most cats. Acquired at an early age, the virus persists in the gut of most cats for the cats entire life without causing any problem. However, in 2-5% of cats at any age, whether they are indoors or out, a spontaneous mutation of the corona virus may occur. The virus then turns into a virulent strain that leaves the intestine of the blood stream where it creates a slowly but inevitably fatal disease. At present, there is no reliable test to tell the difference between normal and mutant corona viruses, so the disease is difficult to diagnose. A newly discovered mutation in the virus protein coat could help explain how the disease occurs and lead to better diagnostic and treatment measure. Until then, prevention remains the best option. In my experience; the FIP vaccine has proven safe and effective and is advisable for all cats.