I adopted two middle-aged (or older?) cats from the same shelter 3.5 years ago. Both were healthy until recently when one developed FIP. Since I adopted them they have had no contact with other cats, aside from whatever happens behind the scenes at the vet's office.
According to this article from the Cornell Veterinary School, a cat can be exposed to the virus (with no symptoms) and develop FIP years later. So it sounds like it's possible that my cat came to me already infected and the disease only manifested now. (It's also possible that the other cat is a carrier who transmitted it to him without getting sick himself.)
When is a cat with future-FIP infectious? Only during an initial exposure to coronavirus (when my cat caught this), or from that time onward? Only after developing FIP? If a cat has been exposed to coronavirus in the past but has never shown symptoms of FIP, can he transmit that coronavirus to other cats?
I am trying to figure out the effect of this on my remaining cat, and whether I can safely adopt another cat to join him. There is no reliable test for FIP other than biopsies, so at best testing can tell us if he's been exposed to coronavirus (the test looks for the antibodies). But my biggest concern is whether he could be infectious. (If he's also infected then, sadly, there's nothing we can do about that. We can only test for coronavirus and coronavirus doesn't always lead to FIP from what I've read.)