Sometime in 1347 a sailing ship moored in a Mediterranean port unwittingly unleashed one of the most destructive pathogens in history. Unloaded with its cargo and passengers were some deadly stowaways: flea-ridden black rats carrying the bubonic plague. It was a scenario played out many times in ports all around Europe, and the results were always the same: Sickness, suffering, and death on what seemed a cataclysmic scale. The years 1347-1351 saw Europe in the terrifying grip of the worst pandemic it had ever suffered: At least one-third of Europe’s population died from what became known as the Black Death.
Most historians agree that it was bubonic plague, a bacterial disease that periodically flared up in Asia and Europe. The so-called Plague of Justinian devastated the Byzantine Empire in the sixth century, killing an estimated 25 million people. After the Black Death, it continued to strike large numbers of Europeans, most notably in London in 1665. The Third Plague Pandemic, the world's last major outbreak, began in the mid-19th century and lasted well into the 20th.
Medieval Europe was at the mercy of many infectious diseases, including dysentery, influenza, measles, and much feared leprosy. But it was the plague that struck the highest note of terror into people’s hearts. During its peak years, the plague spread faster, farther, and with deadlier effect than ever before or since. Its impact fundamentally altered the social, economic, and religious lives of those who survived, scarring the collective consciousness of the entire continent. It seized victims with alarming speed and its horrific ravages were incurable. None were safe as the plague cut down peasants and princes alike, its leveling of social distinctions resonating in the written accounts of the time. It is little wonder that its medieval chroniclers often assume an extravagant and even apocalyptic tone.