Well, in Harry Potter, spells and curses can be evaded (by ducking or sidestepping). The same should hold true for the Avada Kedavra curse. The times I have read about its use (on the Potters, A.D.) it was either used at point blank range or the victim (A.D.) was hardly able to move. When Voldemort tried it again on Harry in "The Goblet of Fire", Harry is able to counter it due to the nature of his and Voldemort's wands, so there must be an instant where this curse can be avoided. Especially with super-speed.
Since magic does affect Superman, magic potions and magic creatures (werewolves, vampires) should be able to do so, too. Oh, and magic (or divine) weapons, too. (Does Wonderwoman's golden lariat work on him? I think so, but I'm not sure here.)
Something that hasn't been mentioned yet is this "aura-of-a red-sun-thing". The aura of a red sun is supposed to make him vulnerable for the duration of exposure. Is this only a pre-crisis concept, or does that still apply?
Personally, I believe that (normal earthern) bacteria and viruses can't affect Superman unless he is redered vulnerable. Because those affecting a human body harm the body's cells, usually by invading them. Poisons that affect human poysiology might be an entirely different matter, though. They don't necessarily harm the cells themselves, but induce some reactions that compel the cells to dysfunction or even harm themselves. And since even super-cells should contain pores (like doors through the membrane) and carriers (something like electric revolving doors in the membrane) pretty much like ours, I suppose they could get in, too.
