Although acquired immune deficiency syndrome (AIDS) was first described in the USA in 1981, there is evidence that individual cases occurred considerably earlier in Central Africa, and serological and virological data show human immunodeficiency virus (HIV) was present in the Democratic Republic of Congo (DRC) as far back as 1959. It is likely that HIV-1 infection in humans was established from cross-species transmission of simian immunodeficiency virus of chimpanzees, but the circumstances surrounding this zoonotic transfer are uncertain. This presentation will review how causality is established in epidemiology, and review the evidence (a putative ecological association) surrounding the hypothesis that early HIV-1 infections were associated with trials of oral polio vaccine (OPV) in the DRC. From an epidemiological standpoint, the OPV hypothesis is not supported by data and the ecological association proposed between OPV use and early HIV/AIDS cases is unconvincing. It is likely that Africa will continue to dominate global HIV and AIDS epidemiology in the near to medium-term future, and that the epidemic will evolve over many decades unless a preventive vaccine becomes widely available.