Within about five years, roughly a third of Europe’s population was dead. What caused the Black Death was a bacterium riding fleas on rodents, but that simple answer hides a more interesting story about how a disease born in the steppes of Central Asia rode the same trade networks that carried silk and spices, arriving in Europe on merchant ships in the fall of 1347 and killing faster than medieval society could comprehend.
What Caused the Black Death, Biologically
The Black Death was caused by the bacterium Yersinia pestis, which normally circulates among wild rodent populations in Central Asia without causing mass die-offs. The bacterium spreads through the bite of infected fleas, most notably the Oriental rat flea, which fed on infected rodents and then transferred the bacteria to new hosts, including the black rats that traveled aboard virtually every merchant ship and caravan of the medieval world. When flea-infested rats died off in large numbers, hungry fleas turned to humans instead, and a routine rodent disease became a human pandemic.
From the Central Asian Steppe to the Black Sea
Genetic and historical evidence points to Central Asia as the plague’s origin point, with climate shifts likely disrupting rodent populations and pushing infected fleas toward human settlements sometime in the early 1340s. From there the disease moved west along the same overland and maritime trade routes that had connected Asia to Europe for centuries, reaching Crimea by 1346. According to the contemporary chronicler Gabriele de’ Mussi, the Mongol army besieging the Genoese trading post of Caffa in Crimea that year was itself ravaged by plague, and catapulted infected corpses over the city walls, though most historians now think trade traffic and infected rats moving in and out of the besieged port were the more decisive route of transmission than the catapulted bodies themselves.
The Ships That Carried the Plague to Europe
Genoese ships fleeing Caffa carried the disease into the Mediterranean, and in October 1347 a fleet from the Black Sea put in at Messina, Sicily, with much of the crew already dead or dying. Sicilian authorities ordered the ships away, but it was too late; the disease had already reached shore. Within weeks it spread to Genoa, Marseille, and Tunis, and by 1348 it had reached Rome, Florence, Paris, and London, moving with the same trade shipping lanes and overland roads that carried grain, wool, and wine across medieval Europe.
How the Disease Actually Spread Person to Person
The plague took three overlapping forms. Bubonic plague, the most common, caused the swollen, blackened lymph nodes, or buboes, that gave the disease its name, and spread primarily through flea bites. Septicemic plague infected the bloodstream directly and was almost always fatal. Pneumonic plague, the most contagious form, settled in the lungs and could pass directly from person to person through coughing, without any fleas or rats required at all, which helps explain how the disease tore through crowded medieval towns and monasteries even after rat populations had been decimated.
The Death Toll and Its Aftermath
Between 1347 and 1352, the Black Death killed an estimated 25 to 30 million people in Europe alone, roughly a third to half of the continent’s population, with comparably devastating outbreaks across the Middle East, North Africa, and Asia. The sudden, catastrophic labor shortage that followed reshaped European economies, giving surviving peasants and laborers new bargaining power over wages and land that helped erode the rigid feudal structures of the medieval period. The disease never fully vanished afterward, recurring in waves for centuries and prompting cities like Ragusa (modern Dubrovnik) to pioneer the practice of quarantine, isolating arriving sailors for 30 to 40 days before they were allowed ashore.