With all respect, ich isn't bacteria, it's a parasite, like fleas---very much like fleas. It's a small swimmer, has a reproductive cycle, goes for a period into the 'lawn', and reinfests. By allowing your tank to lie fallow, you're effectively killing one of its bases, starving it to death. It does help to have immune fish, because sooner or later a mistake brings it in again, unless you qt.
Personally, it's why I didn't bring in a non-goby-blenny-dragonet until my tank had a little age on it, because ich is very rare in those three breeds. But here's a case where the parasite either came in on the blenny, or rode a water droplet into the tank during transfer from his water...I'm assuming this blenny wasn't quarantined, in which case the disease might not have manifested, because blennies rarely manifest it, but it might at least be more dilute.
Ich for fish and red bugs for acropora corals are two of the most maddening plagues we have in the hobby. If you want to get a donnybrook started the in the forum, all you have to do is title a thread "Ich!" and everyone has an experience and a theory. It boils down to: keep your water as healthy as you possibly can, try to get rid of it as thoroughly as you can, try not to keep ich-prone species until your tank has some age and stability; don't overcrowd your tank; feeding garlic doesn't hurt; some swear by ozone---personally I don't use it; and mostly you just try to cope when the stuff turns up.
As a longtime reefer, I stabilize my tank first, keep only ich-resistent fish at first, or generally. I feed garlic whenever I have a new resident, I watch my params like a hawk [I keep corals] and I haven't lost a fish to ich or had to go fallow for, oh, 20 years.
THis isn't, understand, a claim to divine favor or superior virtue, just that a) I've been real lucky b) I look fish over very carefully for any blemish before I buy c) I keep a narrow range of fish [ignore the tang: he's on loan] d) and I watch params. In the great randomness of the universe, occasionally this isn't going to be enough and I'll get the plague, too. I assess how bad it is, what species it's on, and how deep a mess I'm going to be in---if it were, say, on the tang, I'd know I was in deep trouble, and I'd start pulling fish out. If it turned up on a rabbit, say, I'd say to myself, that's usually a short outbreak, I've only got gobies and blennies, and that's not going to be much of a problem, granted my water is really good. At that point I count bumps, and if they start gaining even by one or two in the next 12 hours, I'd treat it as a major problem and go for qt. It's just a hard, hard call how fast to react, but in the case of a new tank, it is definitely time to react fast and in a major way, imho, and that's the advice I'll give. A new owner and a new tank is a combo of early learning-curve and unstable water [nature of any new tank, no matter if Jacques Cousteau set it up] that just makes it safer to pull everybody and go for the Big Cure.