Most diseases are stress mediated. That means that many patogens etc. are already present but they don't bother unstressed fish. However if the fish are stressed the pathogens gain access.

Anything that could be stressing your fish? Poor food supply, rapid temperature changes or low temps for the species (coppernose bluegills florida largemouths in cold water, catch and release in water temps below 50 F., spawning, bad genes from inbreeding, overcrowding, low D.O. levels etc?

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.