this paragraph from the book pretty much sums it up:
"It is seldom realized that most apparently healthy fish are usually carrying a range of potential fish pathogens in or on their bodies. These low-level or latent infections probably do the fish very little harm, but their importance to fish hobbyists cannot be overemphasized. It means that even within the healthiest and best-kept pond or aquarium there lurk pathogens waiting to cause problems for the less-than-vigilant hobbyists. For the most part, caring for the fishes and their environment correctly will allow the natural immune reactions of the fish to keep these pathogens in check. If environmental conditions deteriorate, however, this may reduce the resistance of the fish and result in an outbreak of disease."