Sexually transmitted diseases (STDs) — or sexually transmitted infections (STIs) — are generally acquired by sexual contact. The organisms (bacteria, viruses or parasites) that cause sexually transmitted diseases may pass from person to person in blood, semen, or vaginal and other bodily fluids.

Sometimes these infections can be transmitted non-sexually, such as from mother to infant during pregnancy or childbirth, or through blood transfusions or shared needles. STDs don’t always cause symptoms. It’s possible to contract sexually transmitted diseases from people who seem perfectly healthy and may not even know they have an infection.