The first evidence of human immunodeficiency virus ( HIV ) comes from samples obtained from African patients in the 1950s. It is believed that the first infection occurred in Africa, following the infection from other species of infected primates. In fact, HIV-1 represents the mutated version of a monkey immunodeficiency virus ( SIV ), which has lived with these animals for many hundreds of years. The viral agent (identified as HIV-1 group M) was originally transmitted to humans by chimpanzees .
In fact, this passage would have occurred more than once in central Africa, at the beginning of the twentieth century, but only one of these cases led to the outbreak of the AIDS epidemic among human beings. A study conducted by an international research team, published in "Science", reconstructed the genetic and epidemiological history of AIDS ("The early spread and epidemic of HIV-1 in human populations"; Science 2014 Oct 3; Vol .346 No. 6205 pp. 56-61). Comparing the genetic sequences of hundreds of HIV samples from different parts of Africa, the research group has created a sort of genealogical tree. By applying knowledge on the rate of virus mutations, scholars were able to link the different branches of the family tree with the historical data available. Thus, it was possible to trace the origin of the AIDS epidemic : the virus spread for the first time in 1920 to Kinshasa, the current capital of the Republic of the Congo.
At the beginning of the 20th century, in fact, Kinshasa was a booming colonial city, the center of a flourishing trade; following the expansion of rail traffic, it became one of the best-connected cities in central Africa and it is estimated that hundreds of thousands passed through it at the end of the 40s. The genetic adaptability of the virus, population growth, transport development, prostitution, the presence of other infectious diseases and the habit of using unsterilized syringes in hospitals are all factors that contributed to bringing the virus into the city and in other African countries, therefore in the rest of the world .
