The basic reproductive rate is a measure of the potential for growth of an infectious disease epidemic and depends on the pattern of infectious contacts within the host population, the likelihood of infection being transmitted during a contact, and the duration of infectiousness. These three variables are reviewed along with the surveillance data that records the progress of the epidemic, with an emphasis on the HIV-1 epidemic in heterosexual populations in developing countries. The problems with sentinel surveillance data for HIV infection mean that our knowledge of HIV incidence is poor. However, what is clear is that HIV has spread widely in heterosexual populations in sub-Saharan Africa. The scale of the demographic impact seen here will depend on the local AIDS incubation period with a shorter period generating a more acute demographic impact. Heterogeneity in sexual behaviour is vital to generate a high sexual activity "core group" within which HIV spreads rapidly. How far out of this core group the virus will spread depends on the patterns of mixing within populations. Interventions to reduce the incidence of HIV work through reducing the reproductive rate of the virus. To be efficient, these interventions have to be targeted at those most likely to spread the virus.