STDs/STIs are a group of illnesses that are passed from person to person during sexual intercourse, oral sex, or certain types of sex play. These diseases can be caused by bacteria, viruses, or parasites and are spread through intimate sexual contact involving the penis, vagina, mouth, or anus or contact with any of the membranes that line the urinary and/or genital tracts. STDs/STIs are also called venereal diseases.
Health care providers often use the term "infection" rather than "disease" because it is possible for a person to have no symptoms but still have the infection and require treatment. Scientists have identified more than 20 different STIs.1
Learn more about STDs from the Centers for Disease Control and Prevention website.