Sexually transmitted infections (STIs) are usually spread by having unprotected sexual contact with someone who's infected. STIs are also called sexually transmitted diseases (STDs).

You often can't tell if someone has an STI. Many STIs cause no symptoms. In fact, many people who have an STI don't know it.

That's why you need to be extra careful about STI prevention. Barrier methods of birth control such as condoms and dental dams for oral sex help lower the risk of getting most STIs. If you're sexually active, you can help prevent STIs by having just one sex partner who has sex only with you, called monogamous.