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 they had first encountered in 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 40-60% 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 for the sixth century Justinian Plague. 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, more than the Second World War.
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