Sexually transmitted diseases (STDs), or sexually transmitted infections (STIs), are infections generally acquired by sexual contact. You can get an STI by having unprotected sex with a partner who's infected.

But, you can't always tell if someone is infected, because many STIs cause no symptoms. In fact, many people have an STI without knowing it.

That's why you have to be vigilant about STI prevention. If you're sexually active, it is important to have only one partner, and to talk about your plans to remain exclusive with each other. Barrier methods of birth control, such as condoms, help reduce your risk of getting most STIs.

• Sexual health basics
• Women's sexual health

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Aug. 17, 2016