most fish are wild caught. They can bring in all sorts of disease and parasites into the tank. Methods of catching them can be iffy, some are actually poisoned so they can be caught others are darted. Either method can cause problems it is best to make sure the fish is eating well and not losing weight. Ich is a common parasite that can be difficult treat and some fish are more likely to get it than others.
A couple of stores in my area quarantine fish for 2 weeks all that tells me they don't want to battle some of the diseases in their systems but that doesn't mean it takes care of all the diseases and parasites.
It is better to buy tank raised fish they are more expensive but less likely to have problems. Only a few species are tank raised right now.