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In October 1347, a fleet of Genoese ships docked at Messina in Sicily, and many of the sailors aboard were already dead or dying. Within five years, the disease they carried, now known as plague, had swept across Europe and killed an estimated third or more of its population. The Black Death, as it came to be called, remains the deadliest pandemic recorded in European history.

For centuries, it was assumed that plague spread mainly through rats and their fleas, which carried the bacterium Yersinia pestis from the Asian steppe along trade routes. Recent research has complicated this picture. DNA extracted from the teeth of medieval victims confirmed the bacterium's role, but some epidemiologists argue that rat-borne transmission was too slow to explain how quickly the disease moved. They suggest that human fleas and lice, and the pneumonic form of the disease, which spreads through coughing, must have played a major part in dense cities.

The consequences reshaped society. With so many workers dead, those who survived could demand higher wages, and landlords who refused often found their tenants moving elsewhere. In England, the government responded with a law fixing wages at pre-plague levels, but enforcement proved difficult, and resentment helped fuel the Peasants' Revolt of 1381. Some historians go further and argue that labor scarcity encouraged the adoption of labor-saving technology. This claim is harder to prove, since such inventions also depended on other developments.

Perhaps the most lasting effect was psychological. Contemporary chroniclers describe a world in which neighbors abandoned the sick and priests refused to hear confessions, though modern scholars note that such accounts were written by people eager to portray the catastrophe as the collapse of all order. Surviving records of wills and guild activity suggest that many communities continued to function with remarkable resilience.

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