An eight year study, published in scientific journal Nature, claims the HIV-1 virus that leads to AIDS could have infected humans around 1908 in Africa. Scientists found traces of the HIV-1 genome collected in 1960 from a woman who lived in Léopoldville, presently called Kinshasa, the capital of the Democratic Republic of the Congo. An earlier study had also isolated the virus from a 1959 blood sample of a male from Léopoldville. Study of both the samples and estimate of the rate at which the virus mutates over time has led the researchers to conclude that the human strain could have been around for 100 years.  
The study, co-sponsored by the National Institute of Allergy and Infectious Diseases (NIAID),  part of the U.S. National Institutes of Health, was carried out by Michael Worobey of the University of Arizona in Tucson, Arizona and colleagues from the United States, France, Belgium, Australia, Democratic Republic of the Congo and Denmark.
—Michael Worobey, University of Arizona in TucsonEarlier estimates of this nature had indicated the first infection in humans occurred between 1915 and 1941. The present study pushes the date of the infection back to sometime between 1884 and 1924, with a more focused estimate at 1908. Earlier studies have suggested that HIV-1 virus was spread from chimpanzees to humans in Cameroon.
"Now, for the first time, we have been able to compare two relatively ancient HIV strains. That helped us to calibrate how quickly the virus evolved and make some really robust inferences about when it crossed into humans, how the epidemic grew from that time, and what factors allowed the virus to enter and become a successful human pathogen," Dr. Worobey said.  
"HIV is one of these pathogens that you could almost think of as living on the edge of extinction," Worobey continued. He believes that had HIV not been carried to a city, it may not have survived the jump to humans.
"It means there are things we could do to actually make it so that it doesn't have a chance of spreading," Worobey said.
The first human infection could have happened around the time when the colonial cities were established in Africa. Rapid urbanization in colonial Africa around the beginning of the twentieth century may be responsible for the spread of the AIDS pandemic. It is estimated that several thousand people were infected by the 1960s. Today, HIV infection is reported in 33 million people and has killed 25 million. Researchers opine that an understanding of the origin and pathways for human infection of the virus could help in developing a vaccine to fight it.

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Study estimates first human HIV infection 100 years ago