"I hate FIP more than cancer, because cancer is at least easier to diagnose and has a treatment."
- My vet
The Feline Coronavirus
The feline corona virus is an incredibly common and lackluster virus in cats. The main strand of this virus, the feline enteric coronavirus (FECP) is pretty much unnotable; it is a virus that can chronically infect a cat's intestines and has few outward symptoms. The only symptom that might present is mild diarrhea, and the virus is spread through shared litter trays, through grooming, and even through humans. It's not contagious to humans, but if you pet a cat who has it, and then pet another without, you may spread the virus to the second cat.
Most outdoor cats are exposed to it, and a not-insignificant number of cats are carriers. Because it is spread through contact and feces, it is common in shelters and homes with multiple cats, but again: it's not a big deal in and of itself and causes no discomfort to the cat.
Except sometimes when it mutates.
Feline Infectious Peritonitis
Feline Infectious Peritonitis, AKA FIP, is an uncommon and fatal mutant virus found in some unlucky cats. It occurs when the coronavirus mutates and starts to attack the cat's immune system.
Nobody's entirely sure why some cats get it and some don't. Genetics seem to play a factor, in that if one cat gets it, cats who are related tend to have a higher chance of getting it as well. But the thing about FIP is that it actually isn't contagious per se. The coronavirus is spreadable, but the mutated FIP isn't. What typically happens when there is an outbreak is that the infected cats are related, and so their bodies just happen to mutate the otherwise harmless coronavirus into the same malignant FIP form.
FIP causes fever, lethargy, and weight loss. Due to the change in body chemistry, it can also affect the cat's teeth and gums, giving them tooth rot and very bad breath.
There's still a lot of research being done about FIP and how to combat it, but unfortunately there's not a lot anyone can do. The corona virus mutates frequently, and though most mutations are inconsequential, the mutations that lead to FIP are also regularly changing, which means any time someone comes up with any form of vaccine or treatment, it is rendered obsolete by the time it gets past the first stage trials.
RIP Arcee Summer of 2008- March 2018