Sexually Transmitted Diseases (STDs) or Sexually Transmitted Infections (STIs) or Venereal Diseases: An Overview  Sexually transmitted diseases (STDs) are caused by sexually transmitted infections (STIs). They are spread mainly by sexual contact. Bacteria, viruses, or parasites cause STIs. A sexually transmitted infection may pass from person to person in blood, semen, or vaginal and other bodily fluids. Sometimes sexually transmitted infections are spread in ways other than sexual contact. For example, STIs can spread to infants during pregnancy or childbirth. STIs can also spread through blood transfusions or shared needles. STIs don't always cause symptoms. A person can get sexually transmitted infections from another person who seems healthy and may not even know they have an infection. STDs are also called Venereal Diseases. Sexually Transmitted Infections (STIs) vs. Sexually Transmitted Diseases (STDs): What is the difference between them? Sexually transmitte...