How STIs are Transmitted

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STIs are transmitted by infectious agents—microscopic bacteria, viruses, parasites, fungi, and single-celled organisms called protozoa—that thrive in warm, moist environments in the body, such as the genital area, mouth, and throat. Most STIs spread during sexual intercourse (vaginal or anal), but other forms of sexual contact, such as oral sex, can also spread disease.

Some STIs are transmitted in ways other than by sexual contact. Certain viral STIs, such as AIDS and some types of hepatitis, may be transmitted by contact with infected blood. For instance, viral STIs may pass between people who share infected needles, and a person can become infected from a transfusion of infected blood. Some STIs may pass from an infected mother to her child. Infection may occur before birth, when the infectious agent crosses the placenta (organ in a pregnant woman’s uterus that links the blood supplies of mother and baby) and enters the baby’s bloodstream. Infection also may occur during childbirth, as the baby passes through the birth canal, or after birth, when the baby consumes infected breast milk. STIs cannot be transmitted through shaking hands or other casual contact, or through contact with inanimate objects such as clothing or toilet seats.

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