I'm very sorry!! You must be so worried now!

I thought these tests only show an exposure to a corona virus, and cannot tell if it's a (harmless) entero corona virus or the mutated form, the FIP virus.

I think my vet doesn't like breeders and she thinks that the source IS the breeder.
Well, it's a fact that FIP outbreaks often occur in multicat households, such as breeder's usually have. Stress can be factor for the FECV to mutate to the bad one, FIPV. Also, it's suspected to be inheritable.

But like others here have said before, please try not to worry too much. It doesn't necessarily have to break out! Many cats carry the virus and don't get sick.

Kirsten