Commercials for Herpes medications try to scare you with facts like "70% of people get genital herpes when their partner is showing no signs or symptoms." That's right- you can get herpes not knowing that your partner has it.
But the real question is: what the hell were the other 30% of people thinking? Do you go to sleep with someone, see the bumps, and go "Oh, what's that? Oh well, let's just do it anyways."