How did the Black Death actually spread?
The Black Death of the mid fourteenth century killed a large share of the population of Europe and the Near East. Ancient DNA now identifies the germ, but how it travelled, and what carried it from person to person, is still argued over.
6 established, 5 disputed, 1 debunked, 2 unresolved. We do not pick an answer; the evidence places each claim. How we decide
40 citations from 13 independent sources - 27 echo the same voices and count once. Graded A1-D1.
Added 29 Sept 2026
What the four states mean
- Establishedsupported, uncontested
- Disputedcredible sources disagree
- Debunkedrefuted by the evidence
- Unresolvedno settled answer
Established 6supported, uncontested
Ancient DNA and protein signatures from mass graves in northern, central and southern Europe show that Yersinia pestis caused the Black Death.▰▰▱▱▱ likely2 points
Supports- Here we identified DNA and protein signatures specific for Y. pestis in human skeletons from mass graves in northern, central and southern Europe that were associated archaeologically with the Black Death and subsequent resurgences. We confirm that Y. pestis caused the Black Death and later epidemics on the entire European continent over the course of four centuries.Haensch et al. 2010, Distinct clones of Yersinia pestis caused the Black Death, PLoS Pathogens authoritative
- our data from widely distributed mass plague pits ends the debate about the etiology of the Black Death, and unambiguously demonstrates that Y. pestis was the causative agent of the epidemic plague that devastated Europe during the Middle Ages.Haensch et al. 2010, Distinct clones of Yersinia pestis caused the Black Death, PLoS Pathogens authoritative
Plague can pass to humans through the bite of infected fleas, through unprotected contact with infectious bodily fluids or contaminated materials, and through inhaling respiratory particles from a patient with pneumonic plague.▰▰▱▱▱ likely2 points
Supports- Humans can be infected through: the bite of infected vector fleas unprotected contact with infectious bodily fluids or contaminated materials the inhalation of respiratory particles from a patient with pneumonic plague.World Health Organization, Plague fact sheet analysis
- Any person with pneumonic plague may transmit the disease via respiratory particles to other humans.World Health Organization, Plague fact sheet analysis
At Avignon in 1348, the physician Guy de Chauliac recorded a first form of the mortality with continuous fever and spitting of blood, and a second form with swellings in the armpits and groin, and wrote that it was most contagious in those spitting up blood.▰▰▱▱▱ likely2 points
Supports- Therefore the said mortality began for us [in Avignon, when I was in the service of Pope Clement VI] in the month of January 1348, and lasted seven months. And it took two forms: the first lasted two months, accompanied by continuous fever and a spitting up of blood, and one died within three days. The second lasted the rest of the time, also accompanied by continuous fever and by apostemes [tumors] and antraci [carbuncles] on the external parts, principally under the armpits and in the groin, and one died within five days.Guy de Chauliac, Inventarium sive chirurgia magna (1363), on the 1348 mortality at Avignon, translation in Northeastern University Pandemic Teaching Initiative primary
- And the mortality was so contagious, especially in those who were spitting up blood, that not only did one get it from another by living together, but also by looking at each other, to the point that people died without servants and were buried without priests.Guy de Chauliac, Inventarium sive chirurgia magna (1363), on the 1348 mortality at Avignon, translation in Northeastern University Pandemic Teaching Initiative primary
Boccaccio described the pestilence reaching Florence in the year one thousand three hundred and forty-eight, and described it passing to the healthy through contact with the sick, and even through touching the clothes or things the sick had used.▰▰▱▱▱ likely3 points
Supports- I say, then, that the years [of the era] of the fruitful Incarnation of the Son of God had attained to the number of one thousand three hundred and forty-eight, when into the notable city of Florence, fair over every other of Italy, there came the death-dealing pestilenceGiovanni Boccaccio, The Decameron, Introduction to Day One (John Payne translation), Project Gutenberg primary
- And this pestilence was the more virulent for that, by communication with those who were sick thereof, it gat hold upon the sound, no otherwise than fire upon things dry or greasy, whenas they are brought very near thereunto.Giovanni Boccaccio, The Decameron, Introduction to Day One (John Payne translation), Project Gutenberg primary
- Nay, the mischief was yet greater; for that not only did converse and consortion with the sick give to the sound infection of cause of common death, but the mere touching of the clothes or of whatsoever other thing had been touched or used of the sick appeared of itself to communicate the malady to the toucher.Giovanni Boccaccio, The Decameron, Introduction to Day One (John Payne translation), Project Gutenberg primary
Two cemeteries near Lake Issyk-Kul in modern Kyrgyzstan hold a disproportionally high number of burials dated by tombstone between 1338 and 1339, some inscriptions giving an unspecified pestilence as the cause of death, and ancient DNA from these burials shows a clear involvement of Yersinia pestis in that epidemic.▰▰▱▱▱ likely3 points
Supports- On the basis of tombstone inscriptions, these cemeteries showed a disproportionally high number of burials dating between 1338 and 1339, with some inscriptions stating that the cause of death was due to an unspecified pestilenceSpyrou et al. 2022, The source of the Black Death in fourteenth-century central Eurasia, Nature authoritative
- Here we report ancient DNA data from seven individuals exhumed from two of these cemeteries, Kara-Djigach and Burana. Our synthesis of archaeological, historical and ancient genomic data shows a clear involvement of the plague bacterium Yersinia pestis in this epidemic event.Spyrou et al. 2022, The source of the Black Death in fourteenth-century central Eurasia, Nature authoritative
- Until now, the most debated archaeological evidence potentially associated with the pandemic’s initiation derives from cemeteries located near Lake Issyk-Kul of modern-day KyrgyzstanSpyrou et al. 2022, The source of the Black Death in fourteenth-century central Eurasia, Nature authoritative
The authors of the 2022 Kyrgyzstan study concluded that their data support an early fourteenth-century source of the second plague pandemic in central Eurasia, with a local emergence of the recovered strain in the extended Tian Shan region.▰▰▱▱▱ likely1 source
Supports- Comparisons with present-day diversity from Y. pestis reservoirs in the extended Tian Shan region support a local emergence of the recovered ancient strain. Through multiple lines of evidence, our data support an early fourteenth-century source of the second plague pandemic in central Eurasia.Spyrou et al. 2022, The source of the Black Death in fourteenth-century central Eurasia, Nature authoritative
Disputed 5credible sources disagree
A 2018 modelling study proposed that human ectoparasites such as body lice and human fleas, rather than rats, were the primary vectors of plague during the Second Pandemic, including the Black Death.▰▰▰▱▱ roughly even chance2 against5 points
Supports- Here, we show that human ectoparasites, like body lice and human fleas, might be more likely than rats to have caused the rapidly developing epidemics in pre-Industrial Europe.Dean et al. 2018, Human ectoparasites and the spread of plague in Europe during the Second Pandemic, PNAS authoritative
- Our study supports human ectoparasite transmission of plague during the Second Pandemic, including the Black Death.Dean et al. 2018, Human ectoparasites and the spread of plague in Europe during the Second Pandemic, PNAS authoritative
- Using Bayesian inference, we found that this model fits mortality curves from nine outbreaks in Europe better than models for pneumonic or rodent transmission.Dean et al. 2018, Human ectoparasites and the spread of plague in Europe during the Second Pandemic, PNAS authoritative
Against- However, due to inconsistent modeling choices and reliance on strong assumptions, their analysis does not support their main conclusion.Park, Dushoff, Earn, Poinar and Bolker 2018, Human ectoparasite transmission of the plague during the Second Pandemic is only weakly supported by proposed mathematical models, PNAS letter analysis
- While Dean et al. ( 1 ) show that human ectoparasites could plausibly have been a vector for plague transmission, their conclusion that ectoparasites were likely to have been important is not adequately supported.Park, Dushoff, Earn, Poinar and Bolker 2018, Human ectoparasite transmission of the plague during the Second Pandemic is only weakly supported by proposed mathematical models, PNAS letter analysis
One account of plague reaching Europe has it travelling in rat infestations in the holds of cargo ships, with fleas left by dead rats infecting the humans who unpacked the holds.▰▰▰▱▱ roughly even chance2 against3 points
Supports- Rat infestations in the holds of cargo ships would have been highly susceptible to the rapid spread of plague, and even if most rats died during the voyage, they would have left abundant hungry fleas that would infect humans unpacking the holds.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
Against- While it is commonly assumed that rats and their fleas spread plague during the Second Pandemic, there is little historical and archaeological support for such a claim.Dean et al. 2018, Human ectoparasites and the spread of plague in Europe during the Second Pandemic, PNAS authoritative
- there are no descriptions of rat epizootics, or “rat falls,” that often precede epidemics in the Third PandemicDean et al. 2018, Human ectoparasites and the spread of plague in Europe during the Second Pandemic, PNAS authoritative
A narrative of the siege has fleeing survivors of the siege of Caffa spreading plague from Caffa to the Mediterranean Basin.▰▰▰▱▱ roughly even chance2 against3 points
Supports- This narrative contains some startling assertions: that the Mongol army hurled plague-infected cadavers into the besieged Crimean city of Caffa, thereby transmitting the disease to the inhabitants; and that fleeing survivors of the siege spread plague from Caffa to the Mediterranean Basin.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
Against- The siege of Caffa, and its gruesome finale, thus are unlikely to have been seriously implicated in the transmission of plague from the Black Sea to Europe.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
- Thus, refugees from Caffa would most likely have constituted only one of several streams of infected ships and caravans leaving the region.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
Analysis of medieval Yersinia pestis DNA suggested plague was imported into Europe on two or more occasions, each by a distinct route.▰▰▰▱▱ roughly even chance3 against4 points
Supports- These findings suggest that plague was imported to Europe on two or more occasions, each following a distinct route.Haensch et al. 2010, Distinct clones of Yersinia pestis caused the Black Death, PLoS Pathogens authoritative
Against- we identify that all genomes associated with post-BD outbreaks in Europe derived from a single ancestral strain that was present in southern, central, western and northern Europe during the BD.Spyrou et al. 2019, Phylogeography of the second plague pandemic revealed through analysis of historical Yersinia pestis genomes, Nature Communications authoritative
- pestis genomes reconstructed from Saint-Laurent-de-la-Cabrerisse (southern France) 8 , Barcelona (Spain) 9 , London (England) 10 and Oslo (Norway) 8 were shown to be identical, suggesting the rapid dispersal of a single strain across Europe during the BD.Spyrou et al. 2019, Phylogeography of the second plague pandemic revealed through analysis of historical Yersinia pestis genomes, Nature Communications authoritative
- Our data support an initial entry of the bacterium through eastern Europe, the absence of genetic diversity during the Black Death, and low within-outbreak diversity thereafter.Spyrou et al. 2019, Phylogeography of the second plague pandemic revealed through analysis of historical Yersinia pestis genomes, Nature Communications authoritative
A 2015 climate study found no support for permanent plague reservoirs in medieval Europe, and argued that climate-driven outbreaks in Asian rodent plague reservoirs are associated with new waves of plague arriving in Europe, with repeated reintroductions into European harbours.▰▰▰▱▱ unlikely2 against4 points
Supports- Here, we show that climate-driven outbreaks of Yersinia pestis in Asian rodent plague reservoirs are significantly associated with new waves of plague arriving into Europe through its maritime trade network with Asia.Schmid et al. 2015, Climate-driven introduction of the Black Death and successive plague reintroductions into Europe, PNAS authoritative
- We provide evidence for repeated climate-driven reintroductions of the bacterium into European harbors from reservoirs in Asia, with a delay of 15 ± 1 y. Our analysis finds no support for the existence of permanent plague reservoirs in medieval Europe.Schmid et al. 2015, Climate-driven introduction of the Black Death and successive plague reintroductions into Europe, PNAS authoritative
Against- The lineage identified has not been found in any extant Y. pestis foci sampled to date, and has its ancestry in strains obtained from victims of the 14th century Black Death. These data suggest the existence of a previously uncharacterized historical plague focus that persisted for at least three centuries.Bos et al. 2016, Eighteenth century Yersinia pestis genomes reveal the long-term persistence of an historical plague focus, eLife authoritative
- Analysis of post-Black Death genomes shows the diversification of a Y. pestis lineage into multiple genetically distinct clades that may have given rise to more than one disease reservoir in, or close to, Europe.Spyrou et al. 2019, Phylogeography of the second plague pandemic revealed through analysis of historical Yersinia pestis genomes, Nature Communications authoritative
Debunked 1refuted by the evidence
Research reported in 2001 proposed that the Black Death was not bubonic plague but an Ebola-like virus transmitted directly from person to person.Refuted by
our data from widely distributed mass plague pits ends the debate about the etiology of the Black Death, and unambiguously demonstrates that Y. pestis was the causative agent of the epidemic plague that devastated Europe during the Middle Ages.
refuted1 against3 pointsThe claim rested on- Controversial new research suggests that contrary to the history books, the "Black Death" that devastated medieval Europe was not the bubonic plague, but rather an Ebola-like virus.ABC News, Was Ebola Behind the Black Death? (30 July 2001) summary
- But new research in England suggests the killer was actually an Ebola-like virus transmitted directly from person to person.ABC News, Was Ebola Behind the Black Death? (30 July 2001) summary
Refuted by- our data from widely distributed mass plague pits ends the debate about the etiology of the Black Death, and unambiguously demonstrates that Y. pestis was the causative agent of the epidemic plague that devastated Europe during the Middle Ages.Haensch et al. 2010, Distinct clones of Yersinia pestis caused the Black Death, PLoS Pathogens authoritative
Unresolved 2no settled answer
An account by Gabriele de Mussis of Piacenza of the mortality of 1348 says the besiegers of Caffa ordered corpses to be placed in catapults and lobbed into the city.▰▰▰▱▱ likely4 points
Supports- Here begins an account of the disease or mortality which occurred in 1348, put together by Gabrielem de Mussis of Piacenza.Gabriele de' Mussi, account of the 1348 mortality, as translated in Wheelis 2002, Emerging Infectious Diseases primary
- But they ordered corpses to be placed in catapults 1 and lobbed into the city in the hope that the intolerable stench would kill everyone inside.Gabriele de' Mussi, account of the 1348 mortality, as translated in Wheelis 2002, Emerging Infectious Diseases primary
- After analyzing these claims, I have concluded that it is plausible that the biological attack took place as described and was responsible for infecting the inhabitants of Caffa; however, the event was unimportant in the spread of the plague pandemic.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
- Given the many uncertainties, any conclusion must remain tentative.Mark Wheelis 2002, Biological Warfare at the 1346 Siege of Caffa, Emerging Infectious Diseases analysis
The exact means by which Yersinia pestis travelled from central Eurasia to western Eurasia are unknown, though earlier research suggested warfare and trade networks as main contributors.▰▰▱▱▱ likely1 source
Supports- At present, the exact means through which Y. pestis reached western Eurasia are unknown, primarily due to large pre-existing uncertainties around the historical and ecological contexts of this process. Previous research suggested that both warfare and/or trade networks were some of the main contributors in the spread of Y. pestisSpyrou et al. 2022, The source of the Black Death in fourteenth-century central Eurasia, Nature authoritative
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