Transmission through sexual contact accounts for 75 to 85 percent of the nearly 28 million infections with the human immunodeficiency virus (HIV) that have occurred so far.1 The probability of infection through sexual contact, although it varies greatly, appears to be lower than that of infection through other routes of exposure (Figure 1). The variability observed among and within routes of HIV exposure depends partly on the viral dose and also on whether the virus is transmitted directly into the blood or onto a mucous membrane. In addition, these differences are influenced by a variety of host factors, including both factors common to all routes of exposure and those unique to sexual transmission.
HIV infectivity is the average probability of transmission to another person after that person is exposed to an infected host. Infectivity plus two other parameters — the duration of infectiousness and the average rate at which susceptible people change sexual partners — determines whether the epidemic grows or slows.12 On a population level, all three corners of the classic epidemiologic triangle — host-related factors (susceptibility and infectiousness), environmental factors (the social, cultural, and political milieu), and agent factors (HIV type 1) determine HIV infectivity. Host-related and environmental factors can amplify the epidemic through their dual effect on infectivity and the rate of sexual-partner change. Although the entire triangle is key to understanding infectivity, our article focuses on the epidemiology and biology of the host-related factors that affect the sexual transmission of HIV
