Q: What are the most common ways that HIV is transmitted from one person to another?

A: The three most common ways that HIV is transmitted are:

• By having sex, particularly unprotected sex (anal, vaginal, or oral) with an HIV-infected person
• By sharing needles, syringes, or injection equipment with an intravenous (IV) drug user who is infected with HIV, and
• From HIV-infected women to babies before or during birth, or through breast-feeding after birth (which isn't common in San Francisco).

HIV can also be transmitted through transfusions of infected blood or blood clotting factors. However, since 1985, all donated blood in the United States has been tested for HIV. Therefore, the risk of infection through transfusion of blood or blood products is extremely low. The U.S. blood supply is considered to be among the safest in the world. Some health-care workers have become infected after being stuck with needles containing HIV-infected blood or, less frequently, after infected blood came into contact with a worker's open cut, or through splashes into a worker's eyes, or inside his/her nose, but this is not a common occurrence. There has only been one instance of any patients being infected by an HIV-infected health-care worker. This involved HIV transmission from an infected dentist to six patients (the dentist's instruments weren't properly sterilized).