I thought this fish I had seen at the pet shop might have parasites as it appeared to have something thin and long wiggling around inside of it. When I told one of the workers, they said they would let the manager know. I went back a few days later and the fish was still in there so now I am a bit concerned and paranoid about buying anything from this store.
I wouldn't say that they are common, but they are the unfortunate part of keeping any animal that has been bred elsewhere, shipped (where their immunity really drops and they can become infected easily), and then placed into their final aquarium once purchased from a store that is also holding them in a community styled tank.
I would always suggest asking what treatment any fish has been given once it has arrived at the fish store, and trying to watch for it over a period of time > 5 minutes to make sure that it's acting normal (especially once it comes to eating). As always, I HIGHLY recommend placing the fish into a quarantine tank for a few weeks just to make sure that the fish doesn't show any other illnesses that it may not have displayed at the fish store.
This is something I have been told before. Just have a tank set up separate from the main one so keep fish separated before putting them in. I am considering using my current 5 gal tank to do this when I get a bigger one set up again.