The Black Death killed through three distinct forms of the same bacterium. Bubonic plague attacked the lymph nodes, producing grotesque swellings. Pneumonic plague invaded the lungs. Septicaemic plague poisoned the blood. Each form brought different symptoms, a different speed of death, and different survival rates.
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Date: 1346 to 1353 (first pandemic wave in Europe)
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Causative agent: Yersinia pestis bacterium
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Forms: Bubonic, pneumonic, septicaemic
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Bubonic mortality (untreated): 30 to 80 per cent
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Pneumonic mortality (untreated): Near 100 per cent
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European death toll: Estimated 30 to 60 per cent of population
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Primary medieval sources: Giovanni Boccaccio, Decameron (c.1353); Henry Knighton, Chronica (c.1390); Guy de Chauliac, Chirurgia Magna (c.1363)
The first thing you notice, reading the medieval accounts, is the speed. Not weeks of gradual decline but days. A person who woke up healthy on Monday could be dead by Thursday. Giovanni Boccaccio, who survived the Florence outbreak of 1348 and watched his neighbours fall around him, described it with the blunt precision of a man still processing what he had seen: the disease announced itself quickly, and it announced itself on the body.
That physical announcement is what I want to look at here, because for centuries the symptoms of the Black Death have been either simplified or misrepresented. The popular image is a sufferer covered in dark blotches, clutching a swollen armpit, dying in a ditch. That image is not entirely wrong. But it is also not the whole picture, and the gaps matter enormously. They explain why the plague spread as it did, why some people died within hours while others lasted a week, and why medieval physicians, for all their learning, were so completely unable to stop it.
The Black Death was not one disease in the sense that it expressed itself in one way. It was one bacterium producing at least three distinct clinical presentations. Understanding the difference between them is the key to understanding everything else: how it spread, who it killed first, and what it looked like to the people living through it.
The Bacterium Nobody Knew Was There
Before the symptoms, there is the cause. Yersinia pestis is a rod-shaped bacterium, unremarkable to look at, capable of catastrophic effects on the human body by attacking the immune system's ability to respond to infection. It does this by suppressing the inflammatory response that would normally alert the body to a threat. By the time the immune system mounts a proper defence, the bacterium has already colonised the lymph nodes.
This suppression is what makes Yersinia pestis so lethal. Most bacterial infections announce themselves immediately through fever and visible inflammation. This one delays that announcement. The incubation period for the bubonic form runs between two and six days. During that window, an infected person feels nothing unusual. Then, typically within twenty-four hours of the first symptoms appearing, the signs become unmistakable and progress rapidly.
I find it telling that the bacterium was not identified until 1894, when Alexandre Yersin isolated it during a plague outbreak in Hong Kong. For five and a half centuries, everyone who lived through the Black Death had no framework for understanding what was killing them. They could only describe what they saw. And what they saw was extraordinary in its violence and its speed.
Bubonic Plague: The Swelling That Named the Disease
The most common form was bubonic plague, which likely accounted for the majority of infections during the 1346 to 1353 outbreak. It is the form most people picture, and its defining symptom is the one that eventually gave the disease its common name: the bubo.
A bubo is a swelling of the lymph nodes, driven by the bacterium multiplying inside the node's tissue and triggering an extreme inflammatory response. Buboes appeared most often in the groin, the armpit, and the neck, reflecting the lymphatic pathways from the site of the original flea bite. They could grow to the size of an egg or larger. Medieval sources describe them as hard, intensely painful to the touch, and hot. Some eventually burst and drained. Many did not.
Alongside the bubo, sufferers experienced high fever, chills, severe headache, and a profound exhaustion that made movement difficult. Dark spots could appear on the skin from subcutaneous bleeding, a symptom that connects to one proposed etymology of the name. It is worth noting that "Black Death" was not the term medieval people themselves used: they called it the magna mortalitas, the great mortality. The "blackness" was a later addition, and precisely which blackness it refers to has been debated by scholars ever since.
The mortality rate for untreated bubonic plague is estimated at between 30 and 80 per cent. Those are wide bounds, and they reflect genuine variation in outcomes depending on the individual, the region, and the concentration of infection. Some people recovered. Most did not.
Pneumonic Plague: The Form That Needed No Flea
Pneumonic plague is a different proposition entirely. Where bubonic plague spreads through the bite of an infected flea, pneumonic plague spreads through respiratory droplets. An infected person coughs, and anyone nearby breathing the same air is at risk. No flea required. No rat required. No particular season required.
The symptoms begin much like those of bubonic plague: fever, chills, and weakness. But within two to three days, the lungs become involved. The sufferer develops a cough producing bloody or foamy sputum. Breathing becomes laboured and painful. The pneumonia progresses rapidly, and without treatment it is nearly always fatal.
Medieval accounts describing clusters of deaths in monasteries and close-knit households, where multiple family members died within days of one another rather than weeks, very likely reflect pneumonic transmission. The speed was shocking even by the standards of a pandemic that had already collapsed people's understanding of ordinary mortality.
This form also provides a partial explanation for one of the features of the Black Death that the rat-and-flea transmission model struggles to account for: the pace of spread through certain communities in winter months, when fleas are far less active. Pneumonic transmission needs no insect vector. It needs only proximity and breath, and medieval people lived in extraordinary proximity to one another.
Septicaemic Plague: When the Blood Itself Failed
Septicaemic plague is the rarest and most immediately lethal of the three forms. It occurs when Yersinia pestis enters the bloodstream directly, either from a flea bite or as a complication of the bubonic or pneumonic forms. The bacterium circulates through the blood and triggers a catastrophic septic shock response.
The symptoms are severe and rapid: high fever, extreme weakness, abdominal pain, and bleeding into the skin that produces a purplish or near-black discolouration known as purpura. Gangrene of the extremities was documented in advanced cases: the fingers, toes, nose, and lips turning dark, then black. This is likely the most direct explanation for the "black" element of the name that would eventually attach to the disease, though the naming remains a matter of scholarly debate.
Septicaemic plague could kill within hours of the first symptoms appearing. Some medieval accounts describe people going to bed in apparently good health and being found dead the following morning. Those accounts have often been treated as rhetorical exaggeration, the heightened language of people trying to convey apocalyptic events. Modern understanding of septicaemic plague suggests that some of them were accurate.
What the Medieval Witnesses Recorded
The medieval witnesses to the Black Death were not physicians in the modern sense, but they were careful observers. They wrote what they saw, and what they saw was consistent enough across Europe and England to give us a reliable picture of the symptom patterns even without a bacteriological framework.
Boccaccio remains the most vivid. Writing in the immediate aftermath of the Florence outbreak, he describes the buboes, the dark spots, and the speed of decline. He also captures something that is often overlooked: the social collapse that accompanied the physical symptoms. People who contracted the disease were avoided by their neighbours, left by their servants, sometimes locked inside their houses. Some physicians refused to attend the sick at all.
"In men and women alike it first betrayed itself by the emergence of certain tumours in the groin or the armpits, some of which grew as large as a common apple, others as an egg, some more, some less, which the common folk called gavoccioli."
Giovanni Boccaccio, Decameron, c.1353
Henry Knighton, writing in Leicester in the years after the outbreak, records the devastation of rural villages and the abandonment of livestock by owners who knew they were dying. His account is less medically precise than Boccaccio's but captures something Boccaccio does not: the silence that followed the deaths, whole communities emptied, fields left unploughed, herds wandering without keepers.
Guy de Chauliac, physician to Pope Clement VI at Avignon, recorded two distinct patterns of illness during the 1348 outbreak. One characterised by buboes, lasting approximately five days. The other involving blood-spitting and lasting only two to three days. He was, without knowing it, distinguishing bubonic from pneumonic plague. That distinction would not be formally understood for another five and a half centuries.
If you want to understand what happened to the people who survived these symptoms, what England tried to do to those survivors once the dying stopped, The Price of Survival investigates the thirty years between 1348 and the Peasants' Revolt of 1381: the moment when survivors had to decide what kind of England to rebuild, and powerful interests tried to make that decision for everyone else.
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Why the Symptoms Still Matter
The symptoms of the Black Death are not merely historical curiosities. They are evidence. They tell us how the disease moved through populations, who was most exposed, and why certain communities were destroyed while others, sometimes nearby, survived relatively intact.
The distinction between bubonic and pneumonic plague has direct implications for our understanding of transmission routes. If the rat-and-flea model fully explained the Black Death's spread, the pattern of infection would track rat populations and flea activity fairly closely. In many documented cases, it does not. The pneumonic form, spreading directly from person to person, offers an explanation for outbreaks in cold winters, in high-altitude communities, and in enclosed monastic settings where flea activity would have been extremely limited.
The near-total lethality of the pneumonic and septicaemic forms also helps explain the sheer scale of the mortality. A disease that kills between 30 and 80 per cent of those it infects in its bubonic form, but simultaneously circulates as a pneumonic disease with near-total lethality, is a compound catastrophe. Europe did not lose a third of its population to buboes alone.
What I find most affecting, returning to the primary sources, is the contrast between what the medieval writers tried to make sense of and what they were actually facing. They looked at the symptoms and concluded the disease came from corrupted air, from planetary alignments, from divine punishment. They were wrong about the cause. But their descriptions of the symptoms were precise. And those descriptions, read carefully, still tell us more about the Black Death than many later interpretations.
The survivors of those symptoms, the ones who lived through the fever and the swellings and the loss of almost everyone around them, were left to rebuild a world that had been fundamentally altered. What the English crown did to those survivors is the subject of The Labour Machine: the story of the Statute of Labourers and the state's attempt to control the people it could not afford to lose.
This article is part of The Black Death series. Explore all articles at https://historiesandcastles.com/blogs/the-black-death.
Deepen Your Understanding
Black Death vs Bubonic Plague: 7 Key Differences Explained: How the catastrophic 14th-century pandemic differs from the persistent bacterium that still exists today, and why the distinction matters for understanding what happened in medieval Europe.
Origins of the Black Death: How the Plague Changed Medieval England: From Central Asia to the English coastline: how Yersinia pestis made its way to England and what it found when it arrived.
The Black Death in Medieval England: The sweep of the plague through English towns, villages, and monasteries, and the social order that began to fracture in its wake.
The Bird Man Myth: 7 Surprising Truths About the Plague Doctor's Bizarre Legacy: The physicians who tried to treat these symptoms, what they believed was causing them, and why the iconic bird mask came much later than most people think.
The Black Death: A Catalyst of Social and Economic Change in Medieval Europe: How the symptoms described in this article set in motion a chain of social and economic transformations that remade medieval Europe from the ground up.
The Rat Did Not Do It: 7 Truths About the Black Death That Science Now Confirms: The latest scientific evidence challenging the traditional rat-and-flea model of transmission, and what it means for our understanding of how the plague spread.
People Also Ask
What were the first symptoms of the Black Death?
The first symptoms of the Black Death varied by form. In bubonic plague, the most common form, the earliest signs were fever, chills, headache, and profound weakness, typically appearing two to six days after infection. These were quickly followed by the appearance of buboes: painful swellings of the lymph nodes in the groin, armpit, or neck. In pneumonic plague, early symptoms resembled a severe respiratory illness with fever and coughing. In septicaemic plague, the onset was extremely rapid, with fever and weakness preceding a collapse of the circulatory system within hours.
What did buboes actually look like?
Buboes were swellings of the lymph nodes that formed under the skin, most commonly in the groin, armpit, or neck. Medieval sources describe them growing to the size of an egg or a common apple, though some were smaller. They were described as hard, extremely painful to the touch, and hot. The surrounding skin often appeared reddened. Some buboes eventually burst and released fluid, which contemporary observers noted could lead to recovery in certain cases. Others hardened without bursting, and death typically followed. Boccaccio's Decameron provides one of the most widely cited eyewitness descriptions.
How quickly did people die from the Black Death?
The speed of death depended on which form of plague a person contracted. Bubonic plague typically progressed over three to five days from the appearance of the first bubo to death, though some sufferers recovered. Pneumonic plague moved faster, usually proving fatal within two to three days of the lungs becoming involved. Septicaemic plague was the most rapid: in some documented cases, individuals died within hours of the first symptoms appearing. Medieval accounts of people going to bed healthy and being found dead in the morning are consistent with what modern medicine understands about septicaemic plague.
Did anyone survive the Black Death symptoms?
Yes. Survival was possible, particularly from the bubonic form, which had a mortality rate estimated at between 30 and 80 per cent of those infected. Some individuals recovered after the bubo burst and drained. A small number of people were exposed but showed minimal symptoms. Modern genetic research has identified mutations in the CCR5 gene that appear to confer some resistance to plague, and these mutations occur at higher frequencies in populations descended from regions severely affected by the Black Death. Pneumonic and septicaemic forms were far more lethal, with survival rates approaching zero without treatment.
What is the difference between bubonic and pneumonic plague symptoms?
Bubonic plague and pneumonic plague are caused by the same bacterium, Yersinia pestis, but they present very differently. Bubonic plague produces the distinctive swellings of the lymph nodes known as buboes, alongside fever, chills, and weakness. It spreads through flea bites. Pneumonic plague affects the lungs and spreads through respiratory droplets between people. Its early symptoms resemble bubonic plague, but within two to three days it causes a severe cough producing bloody sputum, respiratory distress, and, without treatment, almost certain death. Pneumonic plague requires no flea or rat for transmission, which partly explains why the Black Death spread so rapidly even in cold weather.
Why was the Black Death called the Black Death?
The term "Black Death" was not used by medieval people themselves, who typically called the pandemic the magna mortalitas (great mortality) or simply "the pestilence." The phrase appeared in historical writing centuries later, and its precise origin is debated. The most commonly cited explanations are: the black or dark discolouration of the skin caused by subcutaneous bleeding and purpura in septicaemic cases; the gangrene of the extremities that darkened fingers, toes, and lips in advanced infections; and a possible derivation from the Latin atra mors, meaning terrible death, where atra was later interpreted as "black" rather than "dreadful."
Primary Sources and Further Reading
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Boccaccio, Giovanni (c.1353): Decameron. The preface to the first day contains Boccaccio's eyewitness account of the Florence outbreak of 1348, including one of the most detailed contemporary descriptions of plague symptoms. Available via Project Gutenberg: https://www.gutenberg.org/ebooks/23700
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Knighton, Henry (c.1390): Chronica Henrici Knighton, Monachi Leycestrensis. The Leicester chronicle includes accounts of the plague's impact on English villages and institutions. Available via the Internet Archive (Rolls Series edition).
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Guy de Chauliac (c.1363): Chirurgia Magna. The account by Pope Clement VI's personal physician at Avignon, distinguishing two patterns of plague illness. Available via WorldCat.
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Ziegler, Philip (1969): The Black Death. Collins. A comprehensive narrative history drawing on contemporary chronicles and later scholarship.
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Aberth, John (2000): From the Brink of the Apocalypse: Confronting Famine, War, Plague, and Death in the Later Middle Ages. Routledge. Available via WorldCat.
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World Health Organization: Plague fact sheet, providing current clinical data on Yersinia pestis and its three forms: https://www.who.int/news-room/fact-sheets/detail/plague
Note: Mortality rate estimates for each form of plague are derived from modern clinical data and retrospective historical analysis. Medieval sources did not distinguish consistently between forms, and population death rates varied significantly by region and chronology. The characterisation of mortality figures here reflects the current range of scholarly estimates rather than a single authoritative figure. The etymology of "Black Death" and the relative contributions of each plague form to overall mortality remain areas of ongoing scholarly discussion.
