St. Roche among the Plague Victims and the Madonna in Glory (1575)
Jacopo Bassano, Public domain, via Wikimedia Commons

In October 1347, the Mongols besieged Kaffa, a city controlled by the Italian city-state of Genoa. To weaken their enemies, they used catapults to launch corpses into the city infected with a deadly disease which had been spreading westward along the Silk Road trade route from China.

After the battle, Genoese ships sailed into Venice carrying soldiers who had been present at the battle. Several of them disembarked complaining of headaches. Within two days, they saw swelling, pus-filled growths called "buboes" appear on their groins and armpits. This was followed by an intense fever and vomiting. Within a week, they were all dead.

By the following summer, bubonic plague had spread across Italy, France, England, Germany, and Scandinavia. Over the next four years, it killed somewhere between one and two thirds of Europe's approximately 80 million people. Further outbreaks struck several times throughout the remainder of the fourteenth century. The European population did not recover for three hundred years.

We now know that the bacterium yersinia pestis was responsible, both for this plague and most likely for earlier outbreaks in the third and sixth centuries. The bacterium is carried inside fleas that cling to rodents, from which they can leap to infect humans and other animals. The disease attacks the lymph nodes, disabling the body's ability to put up an immune response. Thankfully, it can now be easily treated by antibiotics, but in the fourteenth century, it was devastating. Worldwide, it claimed approximately 75,000,000 lives, nearly as many as the Second World War.