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A new, sophisticated analysis of hundreds of genetic sequences of HIV from different time points and locations adds fascinating insights to the origin of the AIDS epidemic. The study, which appears in this issue of Science (see p. 56), confirms earlier analyses that an HIV-infected person came to what today is Kinshasa around 1920, but it then shows for the first time how the virus went from there to two cities in the southeastern portion of the country, likely aided by the extensive rail system that then existed. The researchers also note that 13 documented cases exist of different simian viruses jumping from chimpanzees, gorillas, and monkeys into humans, but only one—known has HIV-1 group M—sparked a global epidemic. They show that group M and another strain, group O, expanded at the same rate until about 1960, but then group M nearly tripled its rate of spread. Possible reasons include public health campaigns that had contaminated needles and an increase in the number of clients of sex workers.