‘What shapes epidemics’ investigates links between HIV/AIDS and certain biological, socioeconomic, and behavioural factors. The main biomedical drivers of HIV/AIDS (after contact with the virus) are the subtype of the viral strain, and the health status and genetic makeup of the exposed individual. Key behavioural factors are the age of sexual debut, sexual practices, number of partners, frequency of partner exchange, concurrency, and mixing patterns including intergenerational sex. People’s behaviour may determine their risk of infection, but behaviours result from the environment in which people live and operate, which is dependent on economics, politics, and culture. Despite the improving technical and biomedical responses, the real challenge is to reduce risk.