Unfortunately, sooner or later, you get the fish that's got it. This is far more likely to happen to a new guy who doesn't know how to assess the overall health of a fish---but even experts get surprised, when the stuff turns out to ride in inside the gills, where its hidden. That's strike one.
Strike two, is when the new guy doesn't recognize it instantly for what it is when it shows up. 24 hour delay figuring it out can be a serious time loss.
Strike three, the new guy has a crowded tank.
Strike four, the water quality is somewhere south of good, and especially the alkalinity and ph are off, which irritates the fish.
Strike five, the new guy, now realizing he has ich, has never had to catch a fish in his tank, and the prolonged chase worsens water quality, stresses everybody, plus he only catches the one fish instead of everybody: his qt isn't large enough for everybody, he doesn't know tricks like eggcrate for dividers, and it just gets to be a sadder and sadder story fast.
You can do variants of this, including the new guy deciding to dose his display tank with some cure, and pretty soon you have somebody quitting the hobby.
This includes the guy with the beautiful huge system who's dodged the bullet for 3 years---and gets the Trojan horse fish that brings it to just about everybody; or the guy who's got no extra room in his apartment who is going to have to live with five hospital/qt tanks for 8 weeks for his largish fish while he starves it out of his display.
PLEASE read the stickies on this topic. It's such a killer, particularly for clowns (who also have their own patented clown disease: brook), tangs, angels, and rabbits (I swear, rabbits are born with it.) And it's so very simple to diagnose and treat if the fish is in qt, and so complicated if it's in your tank.