The more recently acquired Danios came with Fin Rot, my Yellow Tang came with HLLE
(and is just getting over a bacterial infection.. poor guy!), and I've been given Angels (freshwater) which had dropsy.
Admittedly, I did purchase some of the fish from Petco..
Fortunately, the diseases were pretty easily cured (minus the dropsy. I'm still kicking myself).
I'd say I've been pretty lucky - many of the fish came from less than ideal conditions (one, I'm ashamed to admit, came before I had a QT
tank.. fortunately, I know better now and he didn't bring any disease with him).