Probably via text, depending of course, on whether or not you contracted it from him. If you didn't have an STD before you started humping him and all of sudden you have one, then I suggest doing it face-to-face so that you can smart him upside the head with a skillet. Then you can force him to go get tested, etc so that you then have papers to use in court as to why you assaulted him.
Of course, if you have been sleeping around on him, and then contracted an STD, well, I'd suggest that you send him an email telling him to go get tested because you made some bad decisions that possibly gave him the gift that keeps on giving.
Thirdly, did you know you had one before you two started dating? If so, you REALLY should have brought that up before you two decided to make it official. Owning an STD is something that really shold be need-to-know information. While it may make it difficult for you to keep a boyfriend, it's really selfish to NOT tell somebody. And you simply MUST tell him before you become intimate.
Really, this all depends on your situation. But if you're dude gave you the disease where you're forced to ride horses and smile forever (those Herpes ads really did a number on me), you should confront him and don't be apologetic about it. He'll probably claim you're some kind of slore or something and swear he didn't pass it along to you. But as long as you have right on your side, unless your Black, the law will probably look the other way if you do him some bodily harm. And then find out how to fix yourself up.
Quite simply, be straight up, don't sugarcoat s*&^, and keep it thorough. Beating around the bush is what got you two in this situation. Now it's time to extinguish the fire.
Um, no pun intended.
It was written.