I voted that it has never happened to me because I have only had my resident cats get sick from fairly minor illnesses they got from new kitties. But, it was traumatic enough that I am very cautious about letting any new cats near by babies. They are my family and I have to protect them. As much as I admire those who do the riskier rescue work - I simply do not have whatever it takes. If my kitties were to come in contact with any of the serious viral illnesses, I would be a total wreck. Any new cats I adopt must be tested in any way possible and also remain separate for a moderate amount of time.