Ephemerist, Gawker's official sometimes favorite blogger, points out the awesome terror you can inflict with "You've got the AIDS" e-cards, which you send to someone when you're too afraid to own up to possibly giving them an STD. One reads "I got screwed while screwing, you might have too." It's illustrated with a screw! Because laughter is the best medicine, other than antiretroviral therapy.
You can express yourself with five other messages, such as this:
The hand-written note here represents the personal communication that you've avoided in favor of anonymous note on the Internet. Best hope the person you infected is such a slut they don't know who could have sent this.