There is no answer to that question. Can happen any time if there is ick present in the tank and a fish gets stressed. Once in the tank, you have a few different options. My preferred method is not to take the tank apart to enable removing the fish to QT, believing that would stress the fish unreasonably more. Rather, I use a mix of frozen food (either Rod's or M.Y.S.I.S.) with Seachem's Focus and Metronidazole.
My recipe is an adaptation of Seachem's and has worked well, esp in combination with UV (but also worked without). Several others are having luck with that too.
If you search my name, and the words "metronidazole" "focus" and "recipe" you will find a couple relevant threads.