Wow this thread sure blew up!
I've gotten all three major problems on livestock bought locally over the last two years, two on something purchased last week if you want an example (ich / flukes). No LFS around here is going to run them through a full gamut of quarantine for ich, velvet and flukes (not to mention brooklynella, bacterial infections, internal parasites, a myriad of others), for the proper duration and utilizing contamination controls that would also be required (room separated tanks, sanitizing all equipment, etc). At least, not at the prices most people will pay, which is why it doesn't happen.
The OP is having a problem with ich, on whom a tang with a thinner slime coat is more susceptible to gaining larger scale infections. Only fish infected with ich have ich, if quarantined properly, they will not have ich until placed in an environment infected with ich. As for casual observation, you can't even see the ich parasite on a fish, or in any life cycle stage with the naked eye. Only the results of the damage done, the white specs, are visible. As well, most of the parasites will host in the fish's gills rather than on the body, leaving no visual traces.
It took like 2 weeks to learn that from casually reading academic studies, mostly revolving around the wholesale aquaculture (not hobbyist) industry. It's not made up, those business can't afford to make things up when they have millions in food stock potentially at risk of loss from an outbreak. The life cycle, treatment, prevention and ineffective control methods are all well studded and cited in dozens of published journals at this point, probably more well known than any other marine fish disease we'll see in our tanks.
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