From 1347 to 1351, a nightmare disease ravaged Europe, afflicting victims with putrid black boils, fevers, vomiting, and in short order, death. Daily life ground to a halt as the Black Death spread along medieval trade routes, claiming an estimated 20 million lives with ruthless efficiency. Now, a team of researchers is asserting that the plague had an unexpected impact: Clearing the air of a toxic pollutant for the first time in over a thousand years.