In 1966, a 38-year-old man visited a hospital in what is now the Democratic Republic of Congo. His name, his symptoms, and everything about him beyond his age and gender have been lost to history. But a piece of one of his lymph nodes was collected and preserved. By analyzing it, a team of researchers led by Michael Worobey from the University of Arizona have shown that the man was infected by HIV, the virus that causes AIDS. He wouldn’t have known it, though, and nor would his doctors. HIV was formally discovered 17 years later.

By wresting tiny genetic fragments from that tissue sample, Worobey’s team has almost completely reconstructed HIV’s genome from a time before anyone even knew it existed. And that work helps to flesh out the origin story of what would become one of the most important pandemics in human history. “There’s no other way to test these important inferences about the origins of one of the most important infectious diseases to ever hit humans,” says Worobey, who spent about five years trying to piece together that one tiny genome. “In retrospect, we’d probably do it again, but it’s crazy how much work it was.”

HIV was identified only in the 1980s, after a mysterious new syndrome began affecting men in United States. It seemed to come out of nowhere, but it had actually originated decades earlier and a continent away.