HIV can be transmitted through oral, anal, or genital sexual contact. How much of a risk of transmission any given sexual exposure carries depends on what type of contact it is (see Chapter 5 for a description of safe and unsafe sexual practices) and how infectious a partner is. People with HIV are particularly infectious when they are newly infected (and this may even be before a blood test registers as positive) and later in their illness, when they have AIDS. It is at these times that the largest quantities of virus are circulating. However, it is possible to transmit the virus through an unprotected sexual contact regardless of the stage of illness.
A person can become infected from a single sexual contact with an infected partner. On the other hand, some people have repeated exposures to infected partners and either do not become infected at all or become infected only after a period of time. Studies have shown that the risk of a woman acquiring HIV from an infected male partner over a period of years together ranges from 10 to 45 percent, depending on the study. This disparity among results may be due to the use of barrier methods of birth control, how infectious the partners were, genetic differences, or unknown factors. Recent studies have shown that a particular gene may offer protection against becoming infected with HIV however, at this point there is no way that a person can be tested for this gene in the course of routine medical screening.
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