An international team of scientists from the universities of Oxford in the UK and Leuven in Belgium reconstructed the history of the HIV pandemic using historical records and DNA samples of the virus dating back to the late 1950s.

The origin of the pandemic can be traced back to the city of Kinshasa in the 1920s in what is now the Democratic Republic of Congo, but was then Belgian Congo.

The virus is thought to have crossed from chimpanzees to humans in southern Cameroon some time before, but remained a regional infection until it entered Kinshasa (called Leopoldville until 1966), through which one million people were flowing each year using the railways installed by the Belgians.

HIV is a mutated version of a chimpanzee virus, known as simian immunodeficiency virus, which probably made the species-jump through contact with infected blood while handling bush meat.

The virus made the jump on multiple occasions. One event led to HIV-1 subgroup O which affects tens of thousands in Cameroon.

Yet only one cross-species jump, HIV-1 subgroup M, went on to infect millions of people across every country in the world.

More details in the BBC article and in the original paper in Science.