Scientists have uncovered the first Bronze Age animal infected with plague—a 4,000-year-old sheep from the Eurasian steppe. Credit: Shutterstock During the Middle Ages, a devastating plague wiped out roughly one third of Europe's population. The disease spread through fleas that carried the bacterium Yersinia pestis . These fleas passed the infection from rats to humans, fueling the catastrophe known as the Black Death. But the history of plague goes back even further. An earlier form of Y.