Best explanation that I've ever heard is that it's like a virus that's always there. Fish that are healthy and happy are generally able to fight it off. Tangs are the least resistant to it, particularly powders, but any fish that is stressed, underfed, etc, might have a hard time fighting it off. I've heard that some reefers have managed to rid their tank of it, but I'm not sure how they managed that, or how they know that it's been eradicated. Maybe someone will chime in.