The epidemic continues to expand in South America but appears to have plateaued in North America and Europe. The worldwide epidemiology of HIV-1 is a direct reflection of the transmission patterns of the virus. The number of women, African Americans, and Hispanics infected with HIV-1 has grown steadily. In developing countries, the virus is spread primarily through heterosexual contact and affects males and females on a more equal basis than in Western countries. The efficiency of HIV-1 transmission varies with the route, the quantity of virus, and certain phenotypic characteristics of the virus that are incompletely defined.
Sexual contact is a relatively inefficient mode of transmission.
However, it should be emphasized that these are only average risks. Some infected persons appear to be much more efficient than others at transmitting the virus to sexual partners, and some persons remain uninfected despite extensive exposure to the virus through unprotected sexual contacts. Factors accounting for differences among persons with respect to the ability to acquire or transmit HIV-1 have not been fully delineated. It is less clear whether the stage of disease in the transmitting partner plays a strong role in determining the transmission risk. At the outset of the epidemic, concerns were raised that HIV-1 might be transmitted by casual contact in the workplace, at schools, and in nosocomial settings.