HIV-1 first became epidemic during the 1970s in western equatorial Africa, its place of origin. It was at first a silent epidemic, unnoticed until established too firmly to be stopped. In this region, also, during the mid 1980s, the epidemiology of heterosexual HIV/Aids was first determined, exposing a pattern whose main features were to extend throughout sub-Saharan Africa but whose local peculiarities were also to limit epidemic growth within the western equatorial region itself. From this region, moreover, variants of the virus were carried to the rest of the continent.

Although HIV-1 had almost certainly existed in western equatorial Africa since at least the 1950s, it had hitherto struggled even to survive in a sparsely populated region of difficult, often forested environments and poor communications. This was clear from a group of villages at Yambuku in the north of the DR Congo. Blood taken from 659 villagers there in 1976, during one of the first outbreaks of Ebola virus, later revealed that five (0.8 per cent) were infected with HIV. When the villagers were tested again ten years later, HIV prevalence was still 0.8 per cent. Of blood samples collected across the border in southern Sudan in 1976, 0.9 per cent subsequently revealed HIV. Such low levels of infection may well have existed in other rural areas of the equatorial region during the 1970s. They apparently existed also in Kinshasa. One of those testing positive at Yambuku had probably contracted the disease in the capital during the early 1970s. Of 805 blood specimens taken from pregnant women in Kinshasa in 1970, two later revealed HIV infection. So did blood taken there in 1972 from two of four patients with Kaposi's sarcoma.

The conversion of this low-level infection into an expansive epidemic probably took place in the urban environment of Kinshasa during the 1970s. The key may well have been the exceptional infectivity of the newly infected, which meant that if the virus entered a network of sexual relationships in which partners were exchanged rapidly and extensively, it could build up a momentum of infection sufficient to reach epidemic levels.