Angelfish, at least the mass produced ones, tend to have very high parasite and bacteria counts. That's why people say now never to mix angels with discus or festivums, even though in the wild they live together, and in the right aquarium can get along pretty well, too.
Like a lot of "difficult" fish that have now been farmed to be as cheap as possible (also see rams, neon tetras, and dwarf gouramis) a lot of the fish farming practises depend upon the fish being tanked up with hormones and/or antibiotics. As soon as you get them home, then the problems start. The bacteria and parasites are no longer being surpessed by chemicals, and they quickly spread out in the tank, infecting everything else. Some things, like neon tetra disease, will normally only wipe out the neons, but other things, like hole-in-the-head, can do all kinds of damage to many different species.
There's a lot to be said for tracking down wild stock, or at least amateur-bred stock, if you want to really enjoy angels.
Cheers,
Neale