The Black Death was caused by Yersinia pestis and killed millions, reshaping medieval Europe's labor, economy, and daily life.
The Black Death was one of the greatest disasters in medieval history. From 1347 to 1353, an epidemic swept through large parts of Europe, Asia, and North Africa. Entire households lost family members, towns struggled to bury their dead, and surviving workers entered a dramatically changed economy. But the catastrophe was not simply a mysterious curse or a consequence of medieval dirt. It was caused by a specific bacterium: Yersinia pestis.
Modern scientists have identified the pathogen through the study of ancient human remains. Yet an important distinction remains: researchers know what caused plague, while the precise combination of animals, parasites, trade networks, and human contacts that sustained the fourteenth-century pandemic is still being investigated.
What Caused the Black Death, and How Could It Spread?
Yersinia pestis is a bacterium that circulates naturally in some populations of rodents and their fleas. According to the US Centers for Disease Control and Prevention, an infected flea can transmit the bacterium when it bites a person. Contact with infected animals is another possible route. Where plague infects the lungs, close exposure to infectious respiratory droplets can transmit pneumonic plague directly between people.
The disease may take several forms. Bubonic plague often produces painful swellings of the lymph nodes, known as buboes, especially near the groin, armpit, or neck. Septicemic plague involves infection of the bloodstream, while pneumonic plague affects the lungs. Without antibiotics, all three forms can be extremely dangerous. Fourteenth-century Europeans had no knowledge of bacteria and no modern antibiotic treatment.
It is tempting to tell the story as a single chain of infected rats, fleas, and people. That model illustrates a real biological route, but it should not be mistaken for proof that every medieval outbreak followed exactly the same pattern. Scientists continue to investigate the importance of different host animals, flea species, and modes of transmission during the Black Death. A scientific explanation should distinguish established bacterial biology from unresolved details of a particular historical epidemic.
Trade and travel created opportunities for infectious disease to move between communities. Ships, marketplaces, warehouses, and crowded settlements connected distant populations. However, identifying a plausible connection is not the same as reconstructing every medieval journey the bacterium took. The routes and timing of some transmissions remain uncertain.
How Did the Plague Change Work and Society?
When so many people died, the survivors faced a world with fewer farmers, artisans, servants, merchants, and neighbors. The London Museum describes how plague devastated the city after arriving in late 1348. It estimates that roughly 35,000 Londoners had died by 1352, more than half of the population. Two emergency cemeteries, including East Smithfield, were established because normal burial arrangements were overwhelmed.
The immediate consequences extended far beyond grief. With fewer people available to grow food, make goods, or maintain estates, shortages disrupted production. Surviving laborers could sometimes bargain for higher wages, seek different employers, or move where their skills were needed. Some landlords lowered rents to attract tenants. None of those possibilities meant every survivor became wealthy; the effects depended on occupation, region, property, and local rules.
England's government tried to restrain these changes. The Ordinance of Labourers in 1349 and the Statute of Labourers in 1351 sought to regulate employment and limit rising wages. A University of Cambridge study by legal historian Lorren Eldridge explains that these measures also responded to workers' growing mobility and their reluctance to accept undesirable jobs. Enforcement was imperfect, and further outbreaks prompted additional legislation.
The Black Death therefore exposed a struggle over power: people who worked the land or served households could find their labor more valuable, while authorities attempted to preserve older economic relationships. The long-term results were not identical throughout Europe, and the epidemic did not instantly abolish medieval social hierarchies.
What Does New Evidence Reveal—and What Remains Uncertain?
Medieval people often interpreted plague through religion, established medical theories, and the experiences of those around them. Londoners prayed and attended religious observances. Physicians who served wealthier patients could use treatments based on the theory of bodily humors, while ordinary households also relied on herbal preparations. Such responses should be understood in their historical setting, not described as modern germ-based medicine.
For historians, evidence comes from several different kinds of sources. Written accounts can preserve the fear and disruption experienced by contemporaries; burial grounds reveal how communities managed catastrophe; ancient DNA can identify an organism that people at the time could not see. Each kind of evidence answers different questions, and none produces a perfect record of every victim or every journey.
A major advance came in 2022. Researchers publishing in Nature examined remains from cemeteries in the Chüy Valley near Lake Issyk-Kul, in present-day Kyrgyzstan. Gravestones at the sites recorded deaths associated with a pestilence in 1338–1339. The team recovered ancient Y. pestis DNA and found a strain close to the common ancestor of several major plague lineages associated with the pandemic.
That finding strongly supports a Central Eurasian source for the pandemic's early history. It does not prove a single precise starting village or an uninterrupted, documented route from the cemetery to every European port. The paper explicitly notes uncertainty about how the bacterium reached western Eurasia. Historical reconstruction becomes more reliable when it keeps such limits visible rather than filling them with invented certainty.
Mortality figures also require care. The impact varied across places, and surviving medieval records are incomplete. England lost a very large share of its population; London illustrates the scale of an urban catastrophe. But one city's experience should not be presented as a uniform statistic for all of Europe. The consequences likewise unfolded over decades rather than in one sudden, universal social revolution.
Conclusion
The Black Death was caused by Yersinia pestis, a bacterium transmitted through documented animal-associated and, in pneumonic cases, close-contact respiratory routes. Its catastrophic spread in the fourteenth century exposed connected societies to a disease they could neither identify nor effectively treat. Medieval Europe emerged with devastated communities, labor shortages, disputes over wages, and altered relationships between landlords and workers. Ancient DNA and archaeology explain more than ever before, but the most accurate history still distinguishes what is demonstrated from what remains debated.
Sources & Further Reading
- Spyrou et al., “The source of the Black Death in fourteenth-century central Eurasia” (Nature, 2022)
- US CDC, “How Plague Spreads” (2024)
- London Museum, “The Great Pestilence in London”
- Lorren Eldridge, “Centralisation of Labour Regulation in Response to the Black Death” (2025)