In the fall of 1347 a small fleet of Genoese trading ships arrived at the docks of Messina, Sicily. The ships had traveled from Kaffa, a port on the northern reaches of the Black Sea. Since leaving Kaffa, their crews had been struggling against a terrifying, deadly disease. At first, the victims suffered fever, chills, headaches, and nausea. Painful tumors appeared around the lymph nodes in their armpits and groins. Hunger, lethargy, and attacks of mania and panic lasted for several days, followed by death.

The Black Death had arrived in Europe. Before the epidemic of bubonic and pneumonic plague died out three years later, it would kill more than one-third of the continent's population. Entire villages would disappear. Monasteries and manors would be transformed into ghost towns. Serfs would abandon the lands their families had worked for generations, and...