Objectives To understand the factors that drove the exponential spread of HIV-1 in Léopoldville (Kinshasa) in the 1950s.

Methods A review of colonial and post-colonial health service reports, medical publications, and demographic and social science research in Léopoldville.

Results Sex work appeared early in the history of Léopoldville, driven by a strong gender imbalance. Throughout the colonial era, sex work was of a low-risk type, with ‘free women’ having a few regular clients. This sufficed for the persistence of HIV-1, but probably not for the dramatic expansion that occurred in the 1950s. During that decade, genital ulcerative diseases were uncommon and their effect on HIV-1 transmission must have been modest. Circumstantial evidence indicates that this expansion may have been related to parenteral transmission of HIV-1 in the city's sexually transmitted disease clinic, where up to 500 injections were administered daily using syringes and needles that were merely rinsed between patients. Most intravenous injections were given to treat syphilis in patients who never had any clinical evidence of this disease but only had a positive non-treponemal serology, often because of prior yaws infection. An outbreak of ‘inoculation hepatitis’ was reported among these patients in 1951–1952. It is only after the Congo's independence (1960) that, in a context of pauperisation, a pattern of sex work appeared in Léopoldville wherein women had sex with more than 1000 clients each year, allowing the sexual amplification of the virus.

Conclusions It is plausible that the exponential amplification of HIV-1 in Léopoldville occurred mostly parenterally in the 1950s and sexually in the 1960s.