What it was
The Black Death was the second of three major historical plague pandemics caused by the bacterium Yersinia pestis, transmitted primarily by fleas on rats. The first plague pandemic was the Plague of Justinian in the 6th century AD. The third was the modern plague pandemic that began in Yunnan around 1855 and reached its global peak in the early 20th century.
DNA analysis published in 2010 and 2011 by an international team led by Johannes Krause confirmed Y. pestis in skeletons from 14th-century plague pits in London and other European cities, ending centuries of debate about the pathogen.
Where it came from
The most probable origin is in central Asia in the early 1340s — the steppe ecological zone where the natural reservoir of plague-carrying rodents lives. Trade routes carried the disease west; it reached the Crimean Black Sea port of Caffa in 1346, during a Mongol siege that the Genoese chroniclers report involved catapulting plague-infected bodies over the city walls. Genoese ships fleeing Caffa reached Messina in Sicily in October 1347. From Messina, the plague spread by sea and overland across the Mediterranean within months.
How it killed
Plague infection produces three clinical forms. Bubonic plague — the most common, ~80% of cases — produces swollen and blackened lymph nodes (buboes) and a 30–60% mortality rate if untreated. Septicaemic plague enters the bloodstream and is almost universally fatal within hours to days. Pneumonic plague is transmissible from person to person via respiratory droplets and is nearly 100% fatal.
The combination of these three forms, in a population with no acquired immunity and no effective medical response, produced mortality rates unprecedented in European history.
Social consequences
The Black Death produced lasting demographic, economic, religious, and political changes. With 30–60% of the European labor force dead, real wages for the survivors approximately doubled by the late 14th century. The European feudal system, which depended on cheap agricultural labor, never fully recovered. The Catholic Church, which had been the dominant institution responding to the crisis, lost substantial authority — partly because its prayers were visibly ineffective, partly because so many of its clergy died, and partly because of the violent religious movements and antisemitic pogroms the crisis triggered.
The pogroms across Basel, Strasbourg, Cologne, and several hundred other German cities, in which roughly 40,000 to 70,000 Jews were killed, were the largest organised antisemitic violence in Europe before the 20th century.
What followed
Plague recurred in Europe at irregular intervals for the next three centuries — major outbreaks in 1361–1363, 1374, 1400, 1438–1439, 1456–1457, 1464, 1481, 1500–1503, the great Plague of London in 1665, and many smaller events. The third pandemic of the late 19th and early 20th centuries killed approximately 12 million people, predominantly in Asia.
Yersinia pestis still exists. Approximately 2,000 cases of plague are reported worldwide every year, primarily in Madagascar, the Democratic Republic of the Congo, and Peru. The disease is treatable with antibiotics if caught early. Mortality is now under 10% with treatment.