In thirteenth-century Europe, the expansion of royal power brought a gradual end to the chaos and conflict that marked the early medieval period. Urban centers such as London, Paris, and Florence flourished through trade. New industries, such as clothmaking, allowed these cities to grow and a middle class of artisans to prosper. But the relatively peaceful and prosperous times also brought overpopulation. The result was a dwindling supply of food, especially in the overcrowded cities. Poor weather hindered the harvests for several years running, and great famines struck in the first two decades of the fourteenth century, when millions of people, many of them young or elderly, died of starvation and disease.

The contagions and hunger weakened the public's health and resistance to disease, making Europeans vulnerable to an epidemic. By the 1340s the bubonic plague was moving west along the...