The Black Death was a pandemic of bubonic plague — caused by the bacterium Yersinia pestis — that swept through Eurasia from the late 1340s onwards. It arrived in Europe via Sicily in 1347, carried on ships from the Black Sea, and spread with terrifying speed. By 1353, it had killed an estimated 30–60% of Europe's population — somewhere between 25 and 50 million people.
To understand the scale: imagine if between one third and more than half of everyone alive in the UK today died within five years. Not from war or famine, but from a disease that moved from village to village with no way to stop it.
Medieval people had no concept of bacteria or contagion — no germ theory, no understanding of how disease spread. When their neighbours began dying with swollen black lymph nodes, bleeding under the skin, and dark blotches (giving the plague its name), it seemed like the end of the world. Some thought it was divine punishment. Some fled the cities (often carrying the disease with them). Some blamed Jewish communities and carried out pogroms. The total incomprehension of the cause made organised response almost impossible.
How did it spread?
Bubonic plague spreads primarily via fleas on rats. The bacterium lives in the flea's gut; when a flea bites a human, it regurgitates the bacteria into the wound. In medieval Europe, rats were everywhere — in houses, food stores, ships. A pneumonic form (spread by breathing — person to person, far more deadly) also developed. Once pneumonic plague was spreading through a crowded medieval city, it was almost unstoppable.
What were the long-term effects?
Counterintuitively, the Black Death transformed Europe in ways that eventually improved life for survivors. With a third of the population dead, labour became scarce. Peasants who survived could demand better wages and conditions — the feudal system that had kept them in near-serfdom began to crack. It contributed to the end of medieval feudalism and the social upheaval that preceded the Renaissance. Some historians argue the labour shortages that followed accelerated the development of labour-saving technology.