Yes, it’s a freakish bummer. We’ve never had all sick cats before. I hope and pray they get well. Most of them are not willing to be taken to the vet. I know that sounds weird, but if a cat refuses, it’s very difficult, nearly not possible.
Perhaps take one of them, the most sick, or the one least inclined to put up a fight [to the vet], just to see what it might be?