What was the Black Death? The plague that changed Europe

The Black Death was one of the deadliest pandemics in recorded history: an outbreak of plague that reached Europe in 1347 and killed somewhere between a third and a half of its people in about four years. It was caused by the bacterium Yersinia pestis, carried by fleas and spread along the trade routes that linked Asia, the Middle East and Europe. It also struck North Africa and western Asia, and it reshaped work, faith and society in the century after.

In short

• The Black Death was plague, a bacterial disease, and it began in central Asia in the 1330s before reaching Europe by ship in 1347.
• It killed roughly 25 to 50 million Europeans between 1347 and 1351, though the toll varied a great deal from place to place.
• With so many workers gone, wages rose and the old feudal order weakened; fear also drove the murder of thousands of Jews.

What caused the Black Death?

The cause is plague, an infection by the bacterium Yersinia pestis. For a long time this was an educated guess based on medieval descriptions of the swellings victims suffered. It is now settled: researchers have recovered the bacterium's DNA from the teeth of people buried in plague pits of the 1340s, in London and elsewhere.

Plague lives naturally in wild rodents, such as marmots and gerbils, and the fleas that feed on them. People catch it when an infected flea bites them, or by handling infected animals, or, in one form, by breathing in droplets from a sick person's cough. The World Health Organization describes three forms, and medieval writers saw all of them.

Form	What it attacks	How it spreads	Untreated, it kills
Bubonic	The lymph nodes, which swell into painful "buboes"	Flea bites	30% to 60% of cases
Septicemic	The bloodstream	Flea bites, or bubonic plague spreading	Nearly all cases
Pneumonic	The lungs	Coughed droplets, from person to person	Nearly all cases, often within days

Because it is a bacterium and not a virus, plague can today be cured with ordinary antibiotics if they are given early, which is one of the practical differences explained in how viruses and bacteria differ.

Where did it start, and how did it reach Europe?

In 2022, a team led by Maria Spyrou published ancient DNA evidence from two cemeteries near Lake Issyk-Kul in modern Kyrgyzstan, where gravestones dated 1338 and 1339 name "pestilence" as the cause of death. The bacteria in those graves turned out to be the direct ancestor of the strains that later spread west. That places the start of the pandemic in the Tian Shan mountains of central Asia, a region where plague still circulates among wild rodents.

From there it traveled the caravan and sea routes of the Mongol-era trade network. In 1346 it reached Caffa, a Genoese trading port on the Black Sea in Crimea, which an army of the Golden Horde was besieging. One chronicler, Gabriele de' Mussis, wrote that the attackers catapulted plague corpses over the walls; historians still argue whether that happened, and fleas on rats and grain cargoes could have done the job alone.

In October 1347, Genoese galleys from the Black Sea docked at Messina, in Sicily, carrying the disease into Europe. From the Mediterranean ports it moved inland along rivers and roads, at the pace of travelers and goods.

How it spread, year by year

When	Where
1338–1339	Deaths from "pestilence" near Lake Issyk-Kul, central Asia
1346	Caffa, on the Black Sea, and the lands of the Golden Horde
1347	Constantinople, Alexandria, Messina, Marseille
1348	Italy, France, Spain, southern England, Ireland; Florence and Paris hit hard
1349	Germany, the Low Countries, northern England, Norway
1350–1351	Scandinavia, the Baltic and western Russia
1352–1353	Moscow and the far north and east of Europe

How many people did the Black Death kill?

No one counted at the time, so every figure is a reconstruction from tax rolls, church records, manor court records and graves. Most historians put Europe's losses at between a third and a half of its population, which means something like 25 to 50 million people, and some, such as the historian Ole Benedictow, argue for as much as 60%. Popular histories often round it to a third: History.com, for example, gives more than 20 million people, about one in three.

The losses were not even. Some cities lost more than half their people, while other regions seem to have been lightly touched. A 2022 study led by Adam Izdebski used pollen from 261 lake and wetland coring sites across 19 countries to see where farmland was abandoned after 1350. Farming collapsed in places such as Scandinavia, France, western Germany, Greece and central Italy, but carried on or even grew in much of central and eastern Europe, and in Ireland and Iberia. The Black Death was catastrophic in many regions and much milder in some.

Plague also kept coming back. Waves returned to Europe every decade or so for the next 300 years, including the Great Plague of London in 1665, so populations took until around 1500 or later to recover.

Still argued

Two questions remain open. The first is the death toll: estimates for Europe run from about 30% to 60%, and the pollen evidence suggests the average hides huge regional differences. The second is how it spread so fast. The classic account blames black rats and their fleas. A 2018 study in PNAS by Katharine Dean and colleagues modeled nine outbreaks and found that human fleas and body lice, passing plague directly between people, fit the death curves better than rats in seven of them. Others reply that rats must still have carried plague between towns. Most researchers now think more than one route was at work.

How did people respond?

Medieval doctors had no idea of germs. The medical faculty of Paris blamed a conjunction of planets in 1345 and corrupted air, or "miasma". Those who could afford to often fled. Many read the plague as God's punishment. Bands of flagellants walked from town to town whipping themselves in public penance, until Pope Clement VI condemned them in 1349.

Fear also turned into violence. Rumors spread that Jews had poisoned wells, and in 1348 and 1349 Jewish communities across Germany, Switzerland and France were massacred, even though Jews were dying of plague too and the pope said so. In Strasbourg in February 1349, hundreds of Jews were burned. Many survivors fled east to Poland and Lithuania.

Some responses worked. Italian cities set up health boards. In 1377, the port of Ragusa, now Dubrovnik, made arriving ships wait 30 days before landing, a period later stretched to 40 days, quaranta giorni in Italian, which gave English the word quarantine. Isolation, not medicine, was the best tool anyone had for the next 500 years, until germ theory, antibiotics and vaccines changed the fight against infectious disease.

How did the Black Death change Europe?

The most lasting effect was economic. With so many dead, land was plentiful and workers were scarce, so surviving laborers could demand higher pay and better terms. Rulers tried to stop them: England's Ordinance of Labourers in 1349 and Statute of Labourers in 1351 tried to hold wages at pre-plague levels. The pressure built for decades and fed revolts such as England's Peasants' Revolt of 1381. Over time, serfdom, in which peasants were bound to a lord's land, faded in much of western Europe, and ordinary people ate better and earned more than their grandparents had.

The plague also shook trust in the Church, whose prayers had failed and whose priests had died in large numbers, and it left its mark on art, with new images of death, skeletons and the "dance of death". Historians argue about how much of what came later, from the Renaissance to the labor-saving inventions of the 15th century, it can be credited with. It was not the first plague pandemic: the Plague of Justinian struck the eastern Roman Empire in 541. But it is the one by which every later epidemic, including COVID-19, has been measured, and one of the turning points that makes history a story of chance as much as design.

Plague has not disappeared. It still lives in wild rodents in parts of Africa, Asia and the Americas, and most human cases reported to the WHO from 2019 to 2025 were in the Democratic Republic of the Congo and Madagascar. The US Centers for Disease Control and Prevention describes plague as a rare but persistent cause of illness in the rural western United States. This note is history, not medical advice.

Check yourself

• Without looking back: what is the difference between bubonic and pneumonic plague, and why does it matter for how the disease spread?
• Why are death tolls for the Black Death given as wide ranges, and what kinds of evidence are used to estimate them?
• How could a disaster that killed so many people leave the survivors better paid?

Sources

• Plague fact sheet — World Health Organization
• The source of the Black Death in fourteenth-century central Eurasia — Spyrou et al., Nature, 2022
• Palaeoecological data indicates land-use changes across Europe linked to spatial heterogeneity in mortality during the Black Death pandemic — Izdebski et al., Nature Ecology and Evolution, 2022
• Human ectoparasites and the spread of plague in Europe during the Second Pandemic — Dean et al., PNAS, 2018
• Black Death — History.com
• About plague — US Centers for Disease Control and Prevention