Where do STIs come from?

STIs are caused by bacteria, viruses, or parasites that are passed from one person to another during sexual contact.

You catch an STI from someone who already has one. They don't appear out of nowhere. If neither person in a sexual encounter has an STI, neither of you can catch one from each other. STIs are transmitted, not generated.

The infections themselves have been around for a very long time. Some, like syphilis, have been documented for hundreds of years. Others, like HIV, emerged more recently. They're caused by microorganisms (bacteria like chlamydia and gonorrhoea, viruses like HIV and herpes, parasites like trichomoniasis) that evolved to spread through the kind of close physical contact that happens during sex.

The reason STIs spread is that many of them have no symptoms, so people pass them on without knowing. That's why testing matters, even if you feel fine.

STIs are caused by tiny germs like bacteria, viruses, or parasites. They are passed from one person to another during sexual contact.

You catch an STI from someone who already has one. They do not appear out of nowhere. If neither person has an infection, neither can give one to the other.

These infections have been around for a very long time. They spread because many of them have no symptoms, so people pass them on without knowing.

That is why getting tested matters, even if you feel fine.