Clostridium. But another species of Clostridium causes gas gangrene and another causes botulism. Bad genus of organisms.
Used to be viruses were the real nasties.
But these days, there's some new little buggers coming down the pipe that are downright rude.
MSRA staph, some other unwelcome types.
I read a bit about resistant organisms once. It talked about some hospital room, I seem to remember Chicago, where this white stuff started growing on the wall.
They tried all sorts of stuff. Straight Clorox type, peroxide, whatever they use. Actually thought they had it killed off a couple times, but it just kept coming back.
The solution was to back up a truck and fill the whole room. With concrete.
Another one, C. dificil, eats out the gut. Lots of people die from it, in hospitals, though it is sometimes curable...