I'd have to say genital warts or leprosy. Both interfere would with sex, hence, the stuff of nightmares.

MRSA = The flesh eating bacteria. You've got it on your skin all of the time, however should you get an open wound and an active strain. Ooops. This is why you should keep yourselves clean; wash your hands and bathe on a regular basis.

Ultimately, we, those of us in first world countries, particularly the US ar screwed because we've been vaccinated, innoculated, force fed pills since childhood, so when that disease does come a long, be it bacterial or viral, we're screwed. That's why the fucking flu is so fucking virulent. We're resistant to the goddamn shots.

That reminds me. None of you are afraid of the flu. You should be. You should be deathly afraid of the flu. Think of how miserable you are when you've got the flu. Now consider the fact that you figure you're going to survive, but this particular strain, you just don't. This isn't The Stand. This is one season of viral mutation. Swine flu... Bird... take your pick.