In the United States, sexual contact is the most common route of HIV transmission. The CDC has published that of the 42,025 people who they estimated became infected in 2009, 57 percent were men who contracted HIV through sex with other men (MSM). The term MSM is important—and used quite a bit in this lesson—because many men who have sex with men do not necessarily identify themselves as "gay" or even "bisexual." HIV transmitted through sexual activity among heterosexuals accounted for 31 percent of new infections, with most of these cases among women infected by men. Injection drug users, in total, accounted for 12 percent of new infections, though about a quarter of those were MSM, so it isn't possible to know for sure whether those men were infected by sharing injection equipment or through sex.
Heterosexual intercourse is the most common mode of HIV transmission in many resource-poor countries. In Africa slightly more than 80 percent of infections are acquired heterosexually, while mother-to-child transmission and transfusions of contaminated blood account for the remaining infections. In Latin America, most infections are acquired by MSM and through misuse of injected drugs, but heterosexual transmission is rising. Heterosexual contact and injection of drugs are the main modes of HIV transmission in South and South East Asia.
The reason why sexual activity is a risk for HIV transmission is because it allows for the exchange of body fluids. Researchers have consistently found that HIV can be transmitted via blood, semen, and vaginal secretions. It is also true that HIV has been detected in saliva, tears, and urine. However, HIV in these fluids is only found in extremely low concentrations. What's more, there hasn't been a single case of HIV transmission through these fluids reported to the CDC.
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