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.
Some such infections can also 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 — people who, in fact, aren't even aware of being infected. Many STDs cause no symptoms in some people, which is one of the reasons experts prefer the term "sexually transmitted infections" to "sexually transmitted diseases."