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Written by Simon Williams
The Black Death was caused by Yersinia pestis, a bacterium confirmed by ancient DNA extracted from medieval plague victims. How it spread so fast is still debated. The traditional rat-and-flea model has serious problems, and a 2018 study suggested human parasites such as body lice and fleas may have been the primary vehicle.
For a long time, the explanation seemed settled. The Black Death was spread by rat fleas. Infected rodents, mostly the black rat, carried Yersinia pestis in their blood. The rat flea, Xenopsylla cheopis, fed on infected rats and then, when the rat died and the food source ran out, jumped to the nearest warm body. Human. The flea bit. The bacteria entered the bloodstream. The lymph nodes swelled into buboes. Death, in most cases, followed within a week.
It is a compelling story. It is also, increasingly, a story with significant holes in it. The more closely historians and epidemiologists examine the medieval evidence, the less neatly the rat-flea model fits what the sources actually show.
This does not mean the question is settled in the other direction. The Black Death's transmission is one of the most actively debated problems in medieval epidemic history. What I find most intellectually honest about the current state of the scholarship is its willingness to sit with uncertainty rather than defend a tidy orthodoxy.
The first question, at least, is now answered. The causative agent of the Black Death was Yersinia pestis, a bacterium that causes plague in three clinical forms: bubonic (lymph node swellings), septicaemic (blood infection), and pneumonic (lung infection). This was not always beyond dispute. In the late twentieth century, some historians suggested that the Black Death might have been caused by an anthrax-like pathogen, or a viral haemorrhagic fever, on the grounds that the disease spread too fast to be bubonic plague alone.
Ancient DNA analysis settled the debate. Researchers extracted DNA from the teeth and bones of confirmed Black Death victims buried in mass graves across Europe, including sites in London, France, Germany, and Russia. The results were unambiguous: Yersinia pestis was present in the bodies of people who died during the Black Death. A landmark 2011 study confirmed the bacterium across multiple sites, and subsequent research has refined our understanding of the specific strains involved.
The question of what caused the Black Death is therefore closed. The question of how it spread so rapidly across a continent is not.
The traditional transmission model runs like this. Yersinia pestis circulates in populations of wild rodents, particularly rats, in Central Asia. These rodents carry the bacterium without dying from it. When the bacterium jumps to domestic rat populations, those rats lack the immunity of their wild cousins and die rapidly. As infected rats die, their fleas, primarily Xenopsylla cheopis, seek new hosts. Humans become those hosts. The flea bites, regurgitates infected blood, and the bacterium enters the human bloodstream through the bite wound.
This model explains bubonic plague well. It matches the clinical presentation: the swollen inguinal, axillary, or cervical lymph nodes that follow a flea bite on the leg, arm, or neck respectively. It also explains why bubonic plague, when it erupts today in parts of Africa or the American South-West, tends to appear in clusters linked to areas of high rodent density.
The problem is that modern bubonic plague outbreaks spread slowly. They follow the movement of infected rat populations. They are preceded by visible die-offs of rats in affected areas. And they do not typically move at the speed the Black Death moved across medieval Europe, which spread roughly two kilometres per day in some reconstructed epidemic curves.
Medieval chronicles are full of detail about human death. They are notably silent about mass rat mortality. In modern plague outbreaks caused by rat-flea transmission, dead rats in the streets are a well-documented early warning sign. Medieval observers who recorded the plague's progress in vivid detail, noting the smell of the dead, the flight of the clergy, the collapse of trade, do not mention streets full of dead rats. The absence is not conclusive. But it is suggestive.
There is also the matter of northern Europe. The black rat, Rattus rattus, the primary reservoir for Yersinia pestis in the Mediterranean model, is a warm-climate animal. Its presence in medieval Scandinavia and northern Britain is archaeologically patchy. Yet the Black Death swept through Norway, Iceland, and the Scottish Highlands with the same devastating efficiency it showed in Sicily and southern France. If the rat was the essential vehicle, the plague's behaviour in the far north requires additional explanation.
The spread of the Black Death through medieval Europe was an order of magnitude faster than any modern rat-borne plague outbreak. Either the medieval rat population was far more mobile and dense than any modern equivalent, or something else was doing a significant share of the transmission work.
In 2018, Katharine Dean and colleagues published a study in the Proceedings of the National Academy of Sciences that tested three transmission models against historical mortality data from nine European plague outbreaks during the Second Pandemic. The three models were: rat-flea transmission, pneumonic (airborne) transmission, and human ectoparasite transmission, meaning body lice and human fleas.
In seven of the nine outbreaks studied, the human ectoparasite model better matched the observed mortality patterns than the rat-flea model. The study did not claim to have solved the problem. Dean herself was careful to note that mathematical models are not bulletproof evidence, and she anticipated the research would provoke controversy. What it did was shift the burden of proof. If human body lice and human fleas were carrying Yersinia pestis from person to person, the speed of the Black Death becomes considerably more explicable. People move faster than rat populations. People travel in carts, on ships, along trade routes. Their parasites travel with them.
The human ectoparasite model also explains the geographic anomaly of northern Europe more easily. Body lice thrive in cold climates. Populations that rarely washed and wore the same clothes for extended periods, which describes most of medieval Europe, would have carried substantial louse and flea burdens regardless of the ambient rat population.
For readers who want to understand how plague survivors navigated the world the Black Death left behind, The Labour Machine traces the economic and social choices that followed the catastrophe. And for a sharper view of how those who lived through it found ways to endure, The Price of Survival examines the strategies and compromises of the post-plague generation.
While modern researchers debate rats versus lice, the people living through the Black Death had their own explanation: miasma. The theory held that disease was caused by corrupt or putrid air, the invisible effluvium rising from decaying organic matter, swamps, unburied corpses, bad smells. If you breathed miasma, you became ill. If you moved away from the source of the bad air, you improved your chances of survival.
Miasma theory was not irrational within its own framework. It was the best available explanation given the observable facts. The plague did seem to be worse in crowded, filthy places. Moving away from infected areas did sometimes reduce exposure. Burning aromatic herbs, which Miasma theory recommended, would have helped repel fleas. The theory was wrong in its mechanism but accidentally effective in some of its prescriptions.
What Miasma theory could not do was identify the true vector. Nobody looked at fleas. Nobody looked at rats. The plague doctor's famous beaked mask, stuffed with herbs and spices, was designed to filter bad air, not to prevent insect bites. And yet, by covering the face and body in thick waxed leather, some plague doctors may have inadvertently protected themselves from flea bites better than their unmasked colleagues.
One further complication: not all medieval plague was bubonic. Pneumonic plague, caused by the same Yersinia pestis bacterium but affecting the lungs, is directly transmissible between humans through respiratory droplets. It is faster-moving and more lethal than bubonic plague, with a case fatality rate approaching 100 per cent if untreated.
The Black Death almost certainly included outbreaks of pneumonic plague, particularly in winter months when people were crowded indoors and respiratory transmission was easier. Some historians argue that pneumonic plague explains the highest mortality episodes and the fastest geographic spread. The three forms of plague, bubonic, septicaemic, and pneumonic, likely operated simultaneously in different proportions at different times and places during the Second Pandemic.
The honest answer to "how did the Black Death spread?" is therefore: through multiple routes, in proportions that varied by season, geography, and population density, via vectors that modern science has not yet conclusively established. That is less satisfying than a single answer. It is also closer to the truth.
This article is part of the Black Death series. Explore all articles at historiesandcastles.com/blogs/the-black-death.
→ Black Death Symptoms: What the Plague Actually Did to the Human Body. The clinical presentation of bubonic, septicaemic, and pneumonic plague in documented cases, drawing on the medical evidence that survives from the 14th century.
→ The Black Death in Medieval England. How the plague entered England at Weymouth in 1348 and moved through the country, with specific documented mortality figures.
→ Black Death vs Bubonic Plague: 7 Key Differences Explained. A precise comparison of the two terms and why they are not always interchangeable.
→ Plague Doctor Mask: The History Behind the Beak. How contemporary medical theory shaped the response to the plague, including the leather suits and beak masks designed to filter miasma.
→ The Black Death as Catalyst. The social and economic transformation that followed the pandemic, shaped in part by communities' experience of how the disease spread.
The Black Death was caused by Yersinia pestis, a bacterium confirmed by ancient DNA analysis of medieval plague victims. Researchers have extracted Yersinia pestis DNA from the teeth and bones of individuals buried in confirmed Black Death mass graves across Europe, including sites in England, France, and Germany. The bacterium causes three forms of plague: bubonic, which attacks the lymph nodes; septicaemic, which infects the bloodstream; and pneumonic, which affects the lungs. All three forms were probably present during the Second Pandemic of 1347 to 1353.
Rats were long considered the primary reservoir of the Black Death, with the rat flea Xenopsylla cheopis transferring Yersinia pestis to humans when infected rodents died. This model remains valid for many plague outbreaks. However, historians and epidemiologists have raised significant problems with applying it to the medieval Black Death specifically: the plague spread far faster than modern rat-borne outbreaks, medieval sources do not describe mass rat die-offs, and the disease devastated areas such as northern Scandinavia where the black rat was rare. A 2018 study suggested human body lice and fleas may have been the primary vector.
Medieval medical theory attributed the Black Death to miasma: putrid, corrupt air arising from decomposing matter, swamps, or planetary conjunctions. The medical faculty of the University of Paris issued an official report in 1348 blaming a triple conjunction of Saturn, Jupiter, and Mars in Aquarius in 1345, which they believed had corrupted the atmosphere. Treatment therefore focused on avoiding bad air, burning aromatic herbs, and fleeing infected areas. The theory was scientifically incorrect but produced some behaviours, such as fleeing crowded cities and fumigating homes, that may have incidentally reduced exposure.
Pneumonic plague, one of the three forms caused by Yersinia pestis, can spread between humans through respiratory droplets and is directly contagious. It is probable that pneumonic plague occurred alongside bubonic and septicaemic plague during the Black Death, particularly in winter months when people were crowded in enclosed spaces. Some historians argue that pneumonic transmission explains the episodes of highest mortality and fastest spread during the pandemic. However, the relative contribution of pneumonic versus flea-borne transmission to the overall death toll of the Black Death remains a subject of scholarly debate.
The human ectoparasite theory, most fully developed in a 2018 study by Katharine Dean and colleagues, proposes that human body lice and human fleas, rather than rat fleas, were the primary vehicle for Yersinia pestis transmission during the Black Death. Mathematical modelling of historical mortality data from nine European plague outbreaks found that the human ectoparasite model better matched observed patterns in seven of the nine cases. The theory would explain why the Black Death spread faster than modern rat-borne plague, why it reached northern Europe so effectively, and why medieval records do not describe rat die-offs preceding human mortality.
The speed of the Black Death, which moved from Sicily to northern Scotland in roughly three years, has always been difficult to reconcile with a purely rat-borne transmission model. Several factors likely contributed to its velocity: the density and connectivity of medieval trade networks; the possibility that human body lice and fleas, which travel with their human hosts, were primary vectors; the likely concurrent spread of pneumonic plague through respiratory droplets; and the near-total absence of immune resistance in a population that had not encountered the pathogen before. Whatever the precise mechanism, the Black Death was the fastest-moving epidemic in recorded European history before the twentieth century.
Note: The Dean et al. (2018) findings on human ectoparasite transmission represent a significant challenge to the traditional model but are not yet the scholarly consensus. They reflect an active area of debate rather than a settled conclusion. The claim that medieval sources do not describe mass rat die-offs is widely cited in this literature but is itself an argument from silence rather than positive evidence.
Published: 07 July 2026 | Last Updated: 07 July 2026
This episode explores what it was like to live through the Black Death, including how it spread, how people responded, and what it meant for medieval society. Part of the Histories and Castles Deep Dive series.
