What happens when there is an errant mutation / immune system response is that the white cells that are supposed to protect the cat are themselves infected with virus. These infected white cells then carry the virus throughout the cat's body. This results in an inflammation around vessels within the tissues where these infected cells go (abdomen, kidney, or brain).
WHAT SYMPTOMS ARE ASSOCIATED WITH FIP?
Fevers unexplained by anything else are common. In fact cats that develop FIP often appear to have a very sudden onset of symptoms. These then develop quickly and increase in severity over the course of several weeks, ending in death. Otherwise, there are no unique symptoms that unrelated to other illnesses (loss of appetite, weight loss, lethargy / depression, hair / skin problems, fevers).
CAN MY CAT GET FIP?
Any cat that carries the coronavirus can be at potential risk of then developing FIP...However, most often it is immune system compromised cats (kittens, cats infected with feline leukemia, and older cats) that actually develop the disease.
Most cats that develop FIP are less than two years old (cats are most commonly infected as kittens by their infected mothers).
FIP is not highly contagious...however it can be transmitted through cat to cat contact and exposure to feces. If your cat has FIV it is probably better to isolate him from your other cats.
Note that the virus can also live outside in the environment for several weeks so thorough cleaning is important in an area that has housed an infected cat.
The good news is that FIP is not relatively uncommon in the broader cat population. When found it is most commonly located in multiple cat settings like shelters / catteries.
HOW TO PREVENT FIP
Keeping cats healthy and minimizing exposure to infectious agents is the best way to lower the chances a cat will develop FIP. Especially in multiple-cat environments, litter boxes should be kept clean and located away from food and water dishes. Litter should be cleaned daily with the box regularly thoroughly cleaned and disinfected.
Cats suspected having FIP should be separated from other cats.
There is a vaccine against FIP. However it is not generally recommended by the AAFP(American Association of Feline Practitioners) Feline Vaccine Advisory Panel.
The bad news is that there is no good and conclusive test for FIP! This is because cats that carry the coronavirus will test positive. Remember that most cats with the corona virus carry a non-FIP inducing strain or are sufficiently healthy that they will not develop FIP....As a result, a positive test means only that a cat has had a prior exposure to coronavirus and nothing more.
The bad news is that there currently is no known cure or effective treatment for FIP. FIP is a fatal disease. - Simba