Some people might not even know. Sounds disgusting, but true.
One young guy I’m on a sports team with told everyone in the locker-room a story about how he met a skank online and that he got chlamydia from her. He told her, and she said “I don’t have that.”
If he’s one to meet skanks online for sex, perhaps the skank was correct that she didn’t have chlamydia. Maybe he caught it by being a skank with someone else.