It's very easy for something like this to happen. The lady probably was one of the many irresponsible breeders, that let thier cats run wild outside without vaccinations. One of them probbaly contracted it from a wild animal, and spread it to the others. The only real way to test for it is to euthanize the animals, so obviously until they started showing signs no one probably knew. It's too bad that this had to happen, it could have been easily prevented.