I think common sense goes a lot further than worrying when it comes to disease from aquariums. Yes, there are countless bacteria and viruses and other goodies that haven't been studied yet in the ocean (and our aquariums), but I believe proper hygiene and common sense is probably more practical.
If you know you're allergic to everything under the sun (and probably the sun itself...and should just live in a bubble), then chances are, you shouldn't stick your hand in the tank. If you have a gaping wound...chances are, you shouldn't stick your hand in the tank. All I'm saying is, just be careful...but you probably shouldn't lose sleep over it.