Infectious diseases are the terrorists of the natural world.

Most of them, though, quite frankly, aren't very good at inspiring terror. The common cold? An annoyance, but not a terror. The chicken pox? Itchy, but not terrifying. Even a nasty flu is rarely more than acute unpleasantness.

But then there are the few that really stand out -- the diseases that make us all a little more appreciative of our health and a little more respectful of the microbial world.