...Also keep in mind that I believe columnaris can lurk in a tank indefinitely. I've seen this mentioned more than once, but I'm not sure if it is true....However, I've also seen these same statement said of ich, which is definitely not true.
Fungus and bacteria, being such extremely primative lifeforms, are naturally better able to survive harsh conditions (such as drying out entirely) and thus be transferred by their viable reproductive cells hitch-hiking for long periods of time to reach another environment condusive to their propagation. "Ich", or "Ick", (Ichthyophthirius multifiliis
), on the other hand, is a protozoa, and like most (but not all) animals, cannot survive being removed from its normal environment because it does not have a dormant stage that allows it to wait until it ends up in an environment that meets its requirements. (Brine shrimp and fairy shrimp are a strong exception to this generalization.)
Bacteria and fungus often arrive in any body of water-from lake to a glass of water-on a "contaminated" object or simply through the air, and if they find conditions that suit them, they will multiply. (This is why you can establish a proper nitrogen cycle by simply adding something for the bacteria to "eat" then just waiting, without adding a bacterial culture.) So, while the culprits are there, the healthy fish's immune system is constantly fighting them off and keeping them from multiplying to more harmful levels. But when parameters change, even just finally crossing a fine line in water quality, and a fish becomes stressed just enough, then the harmful organism gains a foot-hold and is able to multiply in the one fish first, then, by its new overwhelming numbers, it is able to attack the other exposed fish in the enclosed environment of the aquarium. Wild populations of fish are aided in fighting these diseases by simple dilution of the concentration of the organisms in the more vast waters of their environment, whereas our poor pet fish are trapped "in the same room" with an increasing numbers of their attackers and without our help, soon succumb. This is why an onslaught such as Phoenix-cry can occur so rapidly.