Environmental context, social interactions, and the spread of HIV

The spread of HIV through a population is influenced by many factors. These include the frequency and type of sexual activity, age distribution of the population, use of intravenous drugs and behaviors associated with their use, the pattern of infectivity of the virus during the several stages of the infection, geographic location, and the patterns of interaction among individuals and the context in which those interactions occur. A number of questions about the natural history and epidemiology of AIDS remain unanswered. Since the disease has only recently been recognized, historical data are unavailable to aid in answering these questions. Mathematical models provide one approach that can be of use in understanding the spread of the disease. Because of the complexity of the biology and epidemiology of AIDS, useful models must focus on specific aspects of the disease. In addition to a brief review of existing models for the spread of AIDS, a new model which incorporates the effects of social context operating at the time of a sexual encounter is described. The model considers the spread of the disease in a population of homosexual men divided into groups based on relative risk for the disease. Individuals are assigned to a specific group on the basis of their normal sexual activities, but they may take on the characteristics of a different risk group temporarily given the conditions operating when they engage in a sexual encounter. Results from preliminary analyses of the new model indicate that the major effects of incorporating social context include a decrease in the number of cases of the disease, sometimes by an order of magnitude or more, delayed spread of the disease, and a decreased impact of the disease on low and medium risk groups.