She wrote me this... what do I say?

Wow...she doesn't sound insane, as long as that's what she really said other than the names, times, and places taken out. I wouldn't respond to it or do anything, it would only open her up to throw more of this at you and escalate the situation. If think she may have lost it a little and you think you may be in any kind of danger then I highly recommend that you go to the police. They can advise you further on what you can do to protect yourself and depending on where you live, they may even be able to get an officer out there to help mediate this situation so that things don't get out of hand. She was most likely just venting at you so don't fuel the flames by responding. As far as the STD you should get that checked out as soon as you can and not do anything as far as that's concerned until you find out for certain.