When the bubonic plague arrived in London in 1348, the disease devastated the city. So many people died, so quickly, that the city's cemeteries filled up. "So the king [Edward III], at the time, bought this piece of land and started digging it," says geneticist Luis Barreiro at the University of Chicago. This cemetery, called East Smithfield, became a mass grave, where more than 700 people were buried together. "There's basically layers and layers of bodies one on top of each other," he says. The city shut down the cemetery when the outbreak ended. In the end, this bubonic plague,...