What causes STDs?
Bacteria and viruses that grow in warm, moist places in the body cause STDs. They are passed from one person to another through sex. Infections can spread from the penis, vagina, mouth or anus. These infections can be minor or they can be very painful, even life threatening.
How do sexually transmitted infections spread?
They are spread through fluids in the body. Most often shared during vaginal, oral or anal sex. Some STDs pass from one person to another through infected blood. For example, among people who share infected drug needles. Or a mother may infect her child during pregnancy, childbirth or nursing.
STDs are not spread through casual contact. Shaking hands, sharing clothes, or sharing a toilet seat, for example, would not lead to STDs.
Who is at risk?
Anyone can get a STD. Teenagers and young adults have the highest risk. They are more likely to have many sex partners, and they may not know how to prevent problems. Street-drug users who use dirty needles are also at risk.