Population surveys concerning 'risk behaviours' thought to be related to the AIDS epidemic are many. Nevertheless, unfocused inquiry into diffuse behaviours in undifferentiated populations is not productive in low-seroprevalence populations, especially when the point is to design some form of intervention that might actually avert further infection. This is because of a failure to distinguish conceptually between the relevance of AIDS-related behavioural data for individuals and for populations. An illustration is drawn from the AIDS epidemic in Santiago, Chile, and an alternative perspective, based on extensive interviews with persons with AIDS and a survey of current HIV-surveillance and blood-screening programs, is described.