[Gerald]

No, not every fish exposed to a disease dies from it. If their immune system is healthy and water parameters, diet, etc are good, then they can often fight it off. Early exposure as fry might even have helped. But, i'm wondering if this is columnaris (aka Flexibacter, Flavobacterium). In my experience it's usaully just newly-acquired fish that get it (and sometimes the older fish in the tank with the new ones). I've never seen it reappear after it goes away unless I add new fish. Could it be some other bacterial infection or skin protozoan?