Can't say exactly what I think, as the censorship would kick in, but along the lines of bad cattery. i think it was because they were in a cattery though, and would have been stressed, so their immune system would have been able to let the virus mutate more. One of the sites I posted a link to did say something about catteries. The majority of cats who come into contact with the virus will deal with it and shed it, but it depends on how good their immune system is on how it reacts to the virus, a poor immune system will allow it to mutate slowly, hence dry FIP, and a very poor would allow it to mutate quickly hence wet FIP, which is the worst. The other alternative of course is very bad luck that they had 10 carriers in at one time, but the last stats I Read were about 3% will get FIP, so that really would have to be bad luck.