I believe they are fairly delicate fish. Some seem to be hardier than others, which I'm sure is based largely on breeding and health long before we ever even own the fish.
I believe that I take care of my fish well, they live in very large cycled tanks with plenty of space and water changes, water parameters are frequently checked, they get good food and frequent high quality frozen food, and they *still* come down with stuff. I've got one who seems to have some sort of opportunistic parasite (I've treated him both with salt and with meds...no real change, for the most part, he's fine, but every now and again...he displays symptoms), and another gets fin rot at the drop of a hat. (Double fin in my profile picture) But...I've had others who've survived my own mistaken treatments (prior to coming on here and learning much more than I knew previously), and one who was dying of ammonia poisioning at the petstore that was so bad his scales were sloughing off...But other than being a tail-biter, he's happy, outgoing, and seems to be a very healthy fish after his ordeal.
So, they seem to be a bit disease prone, but they're hardy in that they survive long enough to be treated, they survive the treatment itself, and seem to recover completely or largely completely.
:) And I agree completely with the other people who commented on here that we tend to know more about our preferred breed and their diseases/health, and I think we tend to become more personally involved with them than do many aquarists. Our fish are individuals, not a group in a tank. We comment on and notice their personalities, their quirks, and so when something is not-quite-right, we're immediately aware of it and set out to correct the problem.
Knowing what they survive at many petstores...they are pretty hardy in their ability to live despite awful conditions...but to get them to thrive and be really healthy...then they can suddenly become very fickle and delicate. :)