The Black Death produced three distinct patterns of illness, each with recognizable symptoms that medieval observers documented in detail. The disease, caused by the bacterium Yersinia pestis, could attack the lymph nodes, the lungs, or the bloodstream, and the symptoms varied dramatically depending on which form a person developed. The most iconic sign, the swollen “buboes” that gave bubonic plague its name, was only part of the picture.
Bubonic Plague: Swollen Lymph Nodes and Fever
The most common form of the Black Death was bubonic plague, and its hallmark symptom was the bubo: a painful, swollen lymph node that appeared in the groin, armpit, or neck. These swellings ranged from less than half an inch to about 4 inches across and were extremely tender to the touch. They appeared because the plague bacterium, transmitted through flea bites, traveled through the lymphatic system and triggered a massive inflammatory response at the nearest lymph node.
Symptoms typically appeared 2 to 8 days after exposure. Beyond the buboes, patients developed sudden high fever, chills, headache, and extreme exhaustion. Many experienced nausea and vomiting. As the infection progressed, some people became delirious. The skin around the buboes could darken as tissue began to die, and some buboes ruptured and oozed pus. Without treatment, bubonic plague killed a large majority of those infected, though it was the most survivable of the three forms.
Pneumonic Plague: Coughing Blood
Pneumonic plague attacked the lungs and was the most rapidly lethal form of the disease. It developed in one of two ways: either the infection spread from a bubo into the bloodstream and then to the lungs, or a person inhaled respiratory droplets from someone already infected. This second route made pneumonic plague the only form that spread directly from person to person, which partly explains how the Black Death moved so quickly through crowded medieval cities.
The incubation period could be as short as a single day. Patients developed a high fever, chest pain, difficulty breathing, and a severe cough that produced bloody sputum. The bloody expectoration was a defining feature of this form and would have been immediately alarming to anyone nearby. The disease progressed with terrifying speed. Pneumonic plague was considered virtually 100% fatal without treatment. A pooled analysis of historical cases found a 98% death rate among untreated patients. Even today, survival depends on receiving antibiotics within the first day of illness.
Septicemic Plague: Blackened Skin
Septicemic plague occurred when the bacterium entered the bloodstream directly, without first producing visible buboes. This form accounted for roughly 10% to 15% of all plague cases and was the one most responsible for the disease’s name. As the infection overwhelmed the blood, it triggered a catastrophic chain reaction: the body’s clotting system went haywire, forming tiny blood clots throughout the smallest blood vessels while simultaneously causing uncontrolled bleeding elsewhere.
This process, called disseminated intravascular coagulation, produced the disease’s most visually horrifying symptom. Blood clots blocked circulation to the extremities, causing the fingers, toes, nose, and ears to turn black as tissue died from lack of blood flow. Bleeding under the skin created dark purple or black patches across the body. These gangrenous, blackened extremities are almost certainly where the name “Black Death” originated. The incubation period for septicemic plague is not precisely defined but likely occurred within days of exposure.
Patients also experienced fever, extreme weakness, abdominal pain, and bleeding from the mouth, nose, or under the skin. As the infection progressed, multiple organs began to fail. Septicemic plague was nearly always fatal without treatment, often killing faster than the bubonic form because the bacteria were already circulating freely in the blood.
How Symptoms Overlapped and Progressed
In practice, many victims of the Black Death experienced more than one form of plague simultaneously. A person might develop bubonic plague first, with swollen lymph nodes and fever, only to have the infection spread into the bloodstream (becoming septicemic) and then into the lungs (becoming pneumonic). This progression meant that a single patient could show buboes, blackened extremities, and bloody coughing all at once. Medieval accounts describe exactly this kind of overlapping devastation.
The progression from initial symptoms to death was strikingly fast across all forms. Bubonic plague patients typically deteriorated over several days. Pneumonic and septicemic cases could kill within one to three days. In the advanced stages of any form, tissue destruction and hemorrhage occurred even without the bacteria directly invading new structures. The body’s own inflammatory response caused much of the damage, destroying blood vessels and triggering widespread internal bleeding.
What Medieval Observers Recorded
Fourteenth-century accounts consistently describe the same cluster of symptoms: sudden fever, agonizing swellings in the groin and armpits, dark blotches on the skin, and a foul smell emanating from the sick. Writers noted that some patients died within hours of showing symptoms, while others lingered for days. The speed of death and the visible horror of blackened skin and oozing buboes contributed to the overwhelming terror that accompanied outbreaks.
The constellation of symptoms also made the Black Death unmistakable. Unlike other medieval illnesses, it combined disfiguring swellings, skin discoloration, respiratory distress, and rapid death in a pattern that no other disease replicated. This is why contemporary writers could identify outbreaks with confidence even without any understanding of the bacterium responsible. The symptoms alone told the story.