Sexually transmitted diseases (STDs), or sexually transmitted infections (STIs), are generally acquired by sexual contact. The organisms 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 nonsexually, such as from mother to infant during pregnancy or childbirth, or through blood transfusions or shared needles.
It's possible to contract sexually transmitted diseases from people who seem perfectly healthy, and who may not even be aware of the infection. STDs don't always cause symptoms, which is one of the reasons experts prefer the term "sexually transmitted infections" to "sexually transmitted diseases."