The HIV century

Looks like HIV has been infecting humans since the early 20th century, significantly longer than previous estimates had suggested. It appears that the earliest infections may have been in Kinshasa in the Congo.

So why didn't it seem to become a major issue until the early 1980s? One theory is that the re-use of nonsterile needles during the mid-20th century smallpox vaccination campaigns in Africa may have inadvertently passed HIV directly from person-to-person. Some research indicates that such blood-to-blood transmission can increase the virulence of a pathogen.