In October 1347 twelve Genoese trading galleys put into the port of Messina, on the northeast coast of Sicily, carrying a cargo of sick and dying men. The port authorities ordered the ships back out to sea. It was already too late. Within a month plague was loose in Messina; within six months it was in southern Italy, North Africa, the eastern Adriatic, and Marseille; within two years it had spread across the entirety of western and central Europe; and by the time it burned itself out in 1351 it had killed, by the conservative modern estimate, between a third and half of the European population.
Contemporaries did not call it the Black Death. That name comes from 16th-century Scandinavian chronicles and entered general circulation only in the 18th and 19th centuries. The contemporary term was the Great Mortality (magna mortalitas), the Great Death, or simply the Pestilence. The 14th-century population of Europe before the plague was approximately 75 million; the population in 1400, after the first pandemic wave and several smaller subsequent waves, was approximately 50 million. It would not return to its pre-plague level for almost two hundred years.
Where it came from
The biological cause of the Black Death is now established, after a century of controversy, as the bacterium Yersinia pestis, the same organism that caused the Plague of Justinian in the sixth century and the Third Pandemic that emerged from Yunnan in the 1850s. DNA from Y. pestis has been recovered from medieval plague-burial victims in London, Hereford, Saint-Laurent-de-la-Cabrerisse, Ellwangen, and a dozen other sites. The strain responsible for the 14th-century pandemic is genetically continuous with strains still circulating in modern wild-rodent populations in central Asia.
The most likely origin point, according to recent genomic work led by historians and biologists including Monica H. Green and Maria Spyrou, is the Tian Shan mountains of Kyrgyzstan, where two well-dated cemeteries from 1338–1339 contain victims of an ancestral strain of the responsible bacterium. From there the disease spread westward along central Asian trade routes through the 1340s, reached the Crimea by 1346, and was carried into the Mediterranean on Genoese and Venetian shipping during 1347.
The medieval narrative of the disease’s arrival at Caffa, the Genoese trading colony on the Crimean coast, is partly true and partly legend. The Italian notary Gabriele de’ Mussis, writing in Piacenza in 1348, reported that a Mongol army under Khan Janibeg, besieging Caffa in 1346, catapulted plague-corpses over the walls and that the disease then escaped into the Genoese ships fleeing west. The catapulting story is uncorroborated and may be metaphorical. The general outline — disease at Caffa in 1346, Genoese ships carrying it west in 1347 — is documented from independent sources.
How it killed
The pandemic involved at least two and probably three clinical forms of Y. pestis infection. Bubonic plague, the most common, produces the characteristic bubo — a swollen, blackened lymph node, usually in the groin, armpit, or neck — and is transmitted by flea bite. Pneumonic plague, the secondary respiratory form, is transmitted person-to-person by coughing and is nearly always fatal. Septicemic plague, the rarest, occurs when the bacterium enters the bloodstream directly. Untreated bubonic plague has a case-fatality rate of approximately 50 to 60 percent; pneumonic plague approaches 100 percent.
Modern epidemiological reconstruction suggests that the 14th-century pandemic spread substantially faster than would be expected from rat-flea-mediated bubonic transmission alone. The pneumonic form, the much-debated possibility of human-flea or human-louse transmission, and the simultaneous presence of all three clinical forms in dense urban populations may all have contributed. The result was a disease that could kill an individual within four to seven days of first symptoms and could empty a town within a few weeks.
The standard contemporary account is Giovanni Boccaccio’s introduction to the Decameron, written around 1351 and describing the experience of plague in Florence in 1348. Boccaccio’s Florence buried plague victims in mass pits, abandoned its own sick, and saw its population fall from approximately 100,000 to approximately 50,000 in eighteen months. The Florentine figures are roughly representative of urban Italy and southern France in 1348.
What the survivors did
The pandemic destabilized every European institution. The Church, the medical profession, civic government, the agricultural labor system, the manorial economy, the great fairs, and the long-distance trade routes all broke down to varying degrees during the worst months and then reconstituted themselves under different conditions. The papacy at Avignon under Pope Clement VI issued doctrinal and pastoral guidance during the pandemic and protected the Jewish communities, with limited practical effect. The medical profession’s standard humoral theory produced no effective treatment.
The most visible religious response was the flagellant movement, in which thousands of self-whipping penitents marched through plague-stricken Europe in 1349. The most violent social response was the anti-Jewish pogroms that swept the Rhineland and central Europe in 1348 and 1349, beginning at Basel in January 1349, continuing through Strasbourg on Saint Valentine’s Day, and reaching Cologne in August. The pogroms pre-dated the actual arrival of the plague in many of the affected cities. They had clear precedents in the 1096 Rhineland massacres of the First Crusade.
The long aftermath
The European population reached its 14th-century low point around 1400, after several smaller plague recurrences (1361, 1369, 1374, 1391). The economic consequences were substantial: the surviving labor force could demand higher wages, the rural manorial system began to dissolve in western Europe, and the relative position of agricultural workers improved across most of the continent. The traditional historical claim that the Black Death “ended feudalism” is overstated — feudal relations persisted in much of eastern Europe for centuries — but the western European economy after 1400 was, in measurable respects, a higher-wage and more mobile-labor economy than before.
The pandemic returned in successive waves until the early 18th century. The last major European outbreak was the Great Plague of Marseille in 1720. Yersinia pestis itself has never been eradicated; it continues to circulate in rodent populations in Asia, Africa, and the American west and produces a small number of human cases each year, now treatable with antibiotics.
The Black Death sits as the central demographic event of premodern history, the closest analog to the Great Famine of 1315–1322 which preceded it by a generation, and the model from which subsequent pandemics have been understood. No subsequent disease event has matched its proportional mortality. The 1918 influenza pandemic killed approximately 2.5 percent of the world’s population; the Black Death killed somewhere between thirty and fifty percent of Europe.