C

lostridium difficile. Lookie! A Gram positive organism! It's an anaerobe, which means it doesn't need oxygen. This is ideal for it's environment, which is your intestines. "C. diff" isn't the kind of organism you really want however. It is most closely associated with antibiotic-associated diarrhea. What happens is that, when you're given a dose of antibiotics to treat some other infection, it winds up killing a portion of your normal flora in your intestines. This normal flora typically keeps C. diff in check, however with it now gone, C. diff takes over. Given C. diff's arsenal of toxins, you can then wind up with some nasty diarrhea and severe complications.

There is a rather ... ummm... interesting treatment for people suffering recurrent infections with this organism. Fecal transplants. I only wish I was kidding.

Scanning Electron Micrograph of C. difficile