The Black Death: What Caused It and How It Changed Europe

What caused the Black Death?

The Black Death was caused by plague, an infectious disease of the bacterium Yersinia pestis. The World Health Organization describes Y. pestis as "usually found in small mammals and their fleas," and says it reaches people through the bites of infected fleas, direct contact with infected tissue, and breathing in infected respiratory particles ^[1]. The U.S. Centers for Disease Control and Prevention puts it the same way: people usually catch plague "through the bite of an infected rodent flea or by handling an infected animal" ^[2].

Plague takes three main clinical forms, and the WHO lists them as bubonic, pneumonic and septicaemic ^[1]. Medieval writers described the bubonic form most vividly. The Florentine author Giovanni Boccaccio wrote of "certain swellings, either on the groin or under the armpits," which HISTORY.com explains were swollen lymph nodes ^[8].

Form	What it attacks	Case fatality (WHO)
Bubonic	Lymph nodes, causing painful swellings called buboes	Case-fatality ratio of 30% to 60%
Pneumonic	The lungs; can pass between people through the air	Always fatal if untreated
Septicaemic	The bloodstream	Overall untreated range for plague: 30% to 100%

People in the fourteenth century had no idea a bacterium was to blame; the cause was only identified in the modern era. Today the disease can be cured with antibiotics, the CDC notes, as long as they "are given promptly" ^[2].

Where did the Black Death come from?

The strongest evidence now points to Central Asia. In 2022, a team of geneticists and historians led by Spyrou and Krause published ancient DNA from people buried at two cemeteries, Kara-Djigach and Burana, near Lake Issyk-Kul in modern Kyrgyzstan. Tombstone inscriptions there, "directly dated" to 1338 and 1339, name "pestilence" as the cause of death ^[4].

The genomes the team rebuilt from those graves sit right at the root of a major branching of plague strains linked to the pandemic. The closest living relatives of that strain are found in plague reservoirs of the Tian Shan mountains, and most of those strains have been isolated "from marmots and their ectoparasites." The authors conclude that their data "support an early fourteenth-century source of the second plague pandemic in central Eurasia" ^[4].

That is careful language. The study supports a source region; it does not trace every step of the journey west. HISTORY.com dates the disease's arrival in Europe to October 1347, when "12 ships from the Black Sea docked at the Sicilian port of Messina" ^[8]. Dates before 1582 in this article follow the Julian calendar used at the time.

How did the plague spread across Europe?

Once in Sicily, plague moved fast along trade routes. HISTORY.com lists Marseilles, Tunis, Rome and Florence next, and says that by mid-1348 it had reached Paris, Bordeaux, Lyon and London ^[8]. The National Archives in the United Kingdom records that the second plague pandemic "first came to Britain in 1348" ^[6].

The traditional explanation blames rats and their fleas. That picture is now debated. A 2018 study in the Proceedings of the National Academy of Sciences, led by Dean and colleagues, modeled nine outbreaks of the second pandemic, from Givry in France in 1348 to Malta in 1813. "In seven out of nine localities," the authors found, a model in which human fleas and body lice carried the disease fit the death records better than a rat model ^[5].

The authors are open about the limits. Their models leave out war, famine and public health measures, and they note that transmission from body lice and human fleas to people "has not yet been documented" ^[5]. Rats are no longer the whole story, but the question of exactly how plague jumped from person to person in medieval towns is still open.

How many people died in the Black Death?

No one counted the dead across Europe in the 1340s, so every total is a modern estimate. The figures differ a great deal depending on who makes them and what area they cover.

Estimate	Area covered	Who states it
More than 20 million deaths, "almost one-third" of the population	Europe	HISTORY.com
More than 50 million deaths	Europe	World Health Organization
Up to 60% of the population over eight years	Western Eurasia	Spyrou et al., Nature (2022)

HISTORY.com says the Black Death killed "more than 20 million people in Europe, almost one-third of the continent's population" ^[8]. The WHO fact sheet says plague in the fourteenth century caused "more than 50 million deaths in Europe" ^[1]. The 2022 Nature paper writes that the pandemic is "estimated to have claimed the lives of up to 60%" of the western Eurasian population over its eight-year course ^[4].

These numbers measure slightly different things, so they should not be averaged. What they share is scale. Even the most cautious estimate means roughly one person in three died within a few years.

How did the Black Death change Europe?

The most direct change was economic. With so many people dead, there were far fewer hands to plant, harvest and build. HISTORY.com notes that labor shortages let ordinary laborers demand higher wages and better working conditions, and that people began to question authority, including the Catholic Church ^[8].

England's rulers tried to stop the change by law. The Statute of Labourers of 1351 opens by stating that "a great part of the people and especially of the, workmen and servants has now died in that pestilence," and that some survivors "are not willing to serve unless they receive excessive wages." It ordered workers to accept the pay "accustomed to be paid in the twentieth year of our reign," which means the level from before the plague ^[7]. The National Archives explains that the statute supplemented an ordinance of 1349, issued in a year when Parliament could not meet because of the Black Death, and was "designed to combat rising wages" ^[6].

The law itself is the evidence: rulers would not have tried to freeze wages unless workers were winning higher pay. Historians still debate how far the plague alone ended serfdom, and HISTORY.com reports that some even credit it with paving the way for the Renaissance ^[8]. Those are interpretations, and they compete with other explanations such as trade, war and climate.

The plague also brought fear and violence. Some groups of men formed flagellant processions, traveling from town to town and publicly whipping themselves as penance, until opposition from the pope helped end the movement ^[8]. Far worse, Jewish communities were falsely blamed for the disease, and HISTORY.com reports that "untold thousands of Jews were massacred in 1348 and 1349," while thousands more fled east ^[8]. Like the fall of the Western Roman Empire, the Black Death shows how a crisis can break the bonds that hold a society together.

Is "Ring a Ring o' Roses" really about the plague?

Probably not. The popular story says the "roses" are a rash, the "posies" were carried against bad air, and "we all fall down" means death. But the record does not back it. The earliest printed version of the rhyme appeared in 1881, in a Mother Goose book illustrated by Kate Greenaway, and the first published reading of it as a plague rhyme came in 1961, in James Leasor's book The Plague and the Fire ^[9]. That leaves a gap of five centuries between the Black Death and the rhyme's first appearance in print, and eight more decades before anyone connected the two in writing.

A rhyme can be old without being printed, so the gap alone does not disprove the theory. But a tradition needs evidence before it becomes history, and here there is none. The same caution applies to other popular origin stories, such as the claims made about Halloween's ancient roots.

Does the plague still exist today?

Yes. Plague never disappeared; it retreated into wild animals. The WHO says human plague "remains endemic in several countries," and that most cases reported from 2019 to 2025 came from the Democratic Republic of the Congo and Madagascar ^[1].

In the United States, the CDC reports "an average of five human plague cases" each year, mostly in rural parts of the West: northern New Mexico, northern Arizona and southern Colorado, and California, southern Oregon and far western Nevada. Plague first reached the United States in 1900 on rat-infested steamships, and the last urban epidemic was in Los Angeles in 1924 and 1925 ^[3]. The key difference from 1348 is medicine: with prompt antibiotics, plague can be cured ^[2].

For more stories from this period, browse our Middle Ages era page or the full History Of section. You can also see what happened on any date in On This Day, and our editorial policy explains how we handle death tolls and disputed claims.