Feline coronavirus (FCoV) is a positive-stranded RNA virus that is infectious to cats worldwide. According to Wikipedia , this virus is part of the species Alphacoronavirus 1 of the genus Alphacoronavirus belonging to the virus family Coronaviridae. Alphacoronavirus 1 also includes the canine coronavirus (CCoV) and the porcine transmissible gastroenteritis coronavirus (TGEV). It has two different forms; the FECV (feline enteric coronavirus) that infects the intestines and the FIPV (feline infectious peritonitis virus) that causes the disease feline infectious peritonitis (FIP).
Cornel university says that It generally causes asymptomatic infection but can cause mild diarrhea. As yet poorly understood changes in the virus can give rise to mutants that lead to the development of feline infectious peritonitis (FIP). Most cats infected with an FCoV eliminate virus following infection, but some cats may develop persistent infection. These cats are generally asymptomatic, can shed large amounts of virus in feces, and serve as a continual source of infection for other cats in the environment. A continual circulation of FCoV within a cat population may increase the chance that a virulent FIP strain might emerge. While the pathogenesis of FIP is poorly understood, it is now believed that detection and removal of persistently infected and shedding cats in a multi-cat household can reduce the risk of FIP emergence within that population.
According to https://pets.webmd.com/ Diagnosing FIP is challenging. Despite the claims made by some laboratories and test manufacturers, there is currently no test that can distinguish between the harmless intestinal coronavirus and the deadly FIP coronavirus. A positive test may support the veterinarian’s suspicions, but by itself is inconclusive. It means only that a cat has been exposed to and maybe harboring a coronavirus. A negative test usually (but not always) indicates that the cat is unlikely to have FIP.
If a cat has what appears to be the wet form of the disease, laboratory analysis of some of the fluid can support a diagnosis of FIP. A 1994 study reported that cats with signs suggestive of FIP, who also had a high coronavirus antibody level, reduced numbers of lymphocytes and high levels of globulins in the bloodstream, had an 88.9 percent probability of having FIP. Diagnosing the dry form of the disease is even more challenging, often requiring biopsy of affected organs.