The first part of this paper describes the current global situation of the AIDS pandemic, and the second part the dynamics of the heterosexual transmission of the HIV virus in a population. Risk factors for HIV infection are presented in three different categories: environmental risk factors, including all epidemiological and socio-economic variables which modify the HIV prevalence in a community and operate beyond the control of individuals taken separately; personal risk factors, including the behavioural variables which determine an individual's probability of being exposed to an HIV-positive sexual partner; and biological risk factors, including the known and putative variables which facilitate the transmission of the virus between two partners of differing serostatus. As preventive strategies should be targeted against risk factors which are both preventable and responsible for a large number of HIV transmissions, an attempt is made in this paper to identify in four different populations the risk factors which account for the largest proportion of HIV infections.