We use a deterministic model to study heterosexual HIV transmission. We focus on questions related to sexual partner selection across risk levels and the sensitivity of the model to the differences in infectivity between men and women. We neglect transmission into this purely heterosexual subpopulation from people who have been infected through other means, such as intravenous drug use or sex between men. As well, we neglect age, migration, and many other important features of the epidemic.

Modeling studies have shown that the AIDS epidemic is very sensitive to both the biological aspects of HIV infection and the human behaviors that spread HIV. They have demonstrated that the epidemic is sensitive to subtle features of the biology of HIV and human behavior, including the distribution of times from infection to AIDS, changes in infectiousness with duration of infection, and the distribution of partner acquisition-rates in the population (Hyman and Stanley 1989).

The male and female at-risk populations are divided into uninfected people, those infected with HIV but who have not yet developed AIDS, and the infecteds that have progressed to AIDS. We assume that the major characteristic that affects the probability of infection is the partner-acquisition rate, and distribute each of these populations according to a risk variable which determines this rate. NonAIDS infecteds are also distributed according to their duration of infection, and AIDS cases are distributed according to the duration of time since their diagnosis. People mature into a given risk group. They may change behavior, switching from one risk group to another.