How can I know who gave me a sexually transmitted disease (STD)?

Unless you and your sexual partner were both virgins to sexual activity and neither of you have EVER been outside of your relationship for sexual activity, you cannot know. Some STIs come from an exposure that happened years before. I have counseled married couples who have been faithful throughout the marriage but now one partner has an STI from a premarital exposure. It’s hard for the non-infected partner to believe that his or her partner is still faithful. A weak relationship will be seriously threatened by this event. Prevention is always easier than the cure plus all the fall-out that can happen.