A mysterious new disease first named ‘gay-related immune deficiency’ (GRID) surfaced in the US in 1981. GRID was soon renamed the Acquired Immune Deficiency Syndrome (AIDS), and a viral pathogen, the human immunodeficiency virus (HIV), was identified as its causative agent in 1983. This new illness initially affected mainly marginal groups, such as gay men and injecting drug users in San Francisco and New York or sex workers in Kinshasa. Today, HIV/AIDS is one of the main health and development challenges in large parts of sub-Saharan Africa, affecting men, women and children. Over the past 30 years, HIV/AIDS has gradually revealed itself to be much more than a health issue, with far-reaching social, political and economic implications for individuals, communities and institutions throughout the globe. At the same time, scientists and health professionals have had difficulty identifying the detailed pathways by which the virus is transmitted and designing effective ways to break the cycle of transmission, particularly in areas of high prevalence. Due to these challenging social and biomedical aspects of the disease, HIV/AIDS has been more hotly contested than perhaps any new disease in recent times.