Tell her you have AIDS, and that she might be infected too. Then take her for a test (of course, you have everything rigged), and the doctor tells her she has AIDS. Then have the doctor say, "this looks bad". Then have fun with her the entire day. Have your girlfriend tell her parents and friends. Then, where you are ready, tell here, I was only joking around. If your girlfriend is a good sport, she will laugh, if not, she's too uptight.