The ancestor to HIV may be thousands of years old rather than hundreds, according to a study published in Science and reported by The New York Times. New analysis of the mutation rate in monkeys of the simian immunodeficiency virus (SIV) shows that extant strains of SIV in Africa diverged from a common ancestor between 32,000 and 78,000 years ago. Why such an old virus didn’t spread widely through the human population until the 20th century is unclear. Some researchers attribute it to widespread re-use of mass-produced medical syringes in the 1950s, while others blame the growth of colonial cities and an accompanying increase in sexual contact among multiple partners.

To read the Times article, click here.