The Black Death

Bubonic Plague: Symptoms, Transmission, and Mortality

Bubonic plague was the most common clinical form of the Black Death, accounting for the majority of cases during the fourteenth-century pandemic. Named for the buboes — painful, swollen lymph nodes that are its most distinctive symptom — bubonic plague is transmitted primarily through the bite of infected fleas and represents the classic manifestation of Yersinia pestis infection in humans. Understanding its symptoms, progression, and mortality is essential to understanding the experience of the Black Death.

The Pathogenesis of Bubonic Plague

When an infected flea bites a human host, it regurgitates Yersinia pestis bacteria into the skin. The bacteria are taken up by the lymphatic system and transported to the nearest lymph node, where they multiply rapidly. The lymph node becomes inflamed, swollen, and exquisitely tender — this is the bubo that gives the disease its name.

Buboes typically appear in the groin, armpit, or neck, depending on the site of the flea bite. A bite on the leg produces a bubo in the groin; a bite on the arm produces a bubo in the armpit; a bite on the head or neck produces a bubo in the cervical lymph nodes. The bubo may reach the size of an egg or even an apple, and the overlying skin becomes red, hot, and tense. In some cases, the bubo may suppurate (fill with pus) and rupture spontaneously, discharging foul-smelling fluid.

The bacteria multiply within the lymph node and may overwhelm the local immune defences, spreading into the bloodstream and causing secondary septicaemia. From the blood, the bacteria can reach other organs, including the lungs (causing secondary pneumonic plague), the liver, the spleen, and the brain. The progression from localised lymph node infection to systemic disease determines the severity and outcome of the illness.

Symptoms and Clinical Course

The incubation period of bubonic plague is typically two to six days, though it can range from one to ten days. The onset of symptoms is sudden and dramatic. The patient develops a high fever (often exceeding 40°C / 104°F), severe headache, muscle pain, weakness, and dizziness. The bubo appears within one to two days of the onset of fever and becomes increasingly painful and swollen.

Contemporary descriptions of the Black Death consistently mention the buboes as the most characteristic sign of the disease. Boccaccio, in the introduction to the Decameron, described them as “certain swellings in the groin or under the armpits, some of which grew to the size of an ordinary apple and others to the size of an egg.” The physician Guy de Chauliac noted that the buboes were accompanied by fever and that patients typically died within five days of their appearance.

As the disease progresses, the patient may develop additional symptoms: nausea and vomiting, abdominal pain, diarrhoea, confusion, and delirium. The skin may develop dark patches or blotches — the “tokens” or “God’s tokens” mentioned in contemporary sources — caused by bleeding beneath the skin (subcutaneous haemorrhage). These dark patches may have contributed to the name “Black Death,” though the etymology of the term is debated.

In patients who survive, the bubo may gradually resolve over a period of weeks. In those who die, death typically occurs within three to seven days of the onset of symptoms, usually from septic shock, disseminated intravascular coagulation, or multi-organ failure.

Mortality Rates

The mortality rate of untreated bubonic plague is approximately thirty to sixty percent. This means that even without modern antibiotics, a significant proportion of patients survived the infection. The factors that determined survival are not fully understood but likely include the patient’s age, nutritional status, immune competence, and the size of the initial bacterial inoculum.

During the Black Death, the overall mortality rate — including all forms of plague — was far higher than the thirty to sixty percent mortality of bubonic plague alone. This is because the pandemic involved the co-circulation of pneumonic plague and septicemic plague, both of which have much higher mortality rates. The combined effect of all three forms, operating in populations weakened by malnutrition and other diseases, produced the catastrophic death tolls that characterised the pandemic.

The mortality rate of bubonic plague varied by location, social class, and individual circumstances. Urban populations, living in crowded conditions with high rat and flea densities, were more heavily exposed than rural communities. The poor, who could not afford to flee infected areas or to access the best available medical treatments, suffered higher mortality than the wealthy. Children and the elderly were more vulnerable than healthy adults.

Diagnosis in the Fourteenth Century

Medieval physicians diagnosed bubonic plague primarily through the observation of buboes. The appearance of painful swellings in the groin, armpit, or neck, accompanied by fever and rapid deterioration, was sufficient to identify the disease. The physician Guy de Chauliac distinguished between the form of plague with buboes (which he associated with a somewhat longer course) and the form with spitting of blood (which he associated with rapid death), corresponding to the modern distinction between bubonic and pneumonic plague.

The humoral framework within which medieval physicians worked led them to interpret the buboes as the body’s attempt to expel corrupt humours through the lymph nodes. The treatment — bloodletting, lancing of buboes, and the application of poultices — was designed to assist this process. None of these treatments addressed the underlying bacterial infection, and some (particularly bloodletting) may have worsened the patient’s condition.

Bubonic Plague in Modern Medicine

The development of antibiotics in the twentieth century transformed the prognosis of bubonic plague. Streptomycin, the first effective antibiotic against Yersinia pestis, was introduced in the 1940s and reduced mortality from thirty to sixty percent to approximately five to fifteen percent when administered promptly. Modern treatment typically involves streptomycin or gentamicin, supplemented by doxycycline or ciprofloxacin. With early diagnosis and treatment, bubonic plague is now a manageable condition with a high survival rate.

The modern understanding of bubonic plague, based on bacteriology, immunology, and molecular biology, has revealed the mechanisms by which Yersinia pestis evades the immune system, multiplies in the lymph nodes, and causes systemic disease. This understanding has informed the development of diagnostic tests, vaccines, and treatment protocols that have made plague a treatable rather than a catastrophic disease.

However, bubonic plague remains a significant public health concern in parts of the world where it is endemic, particularly in Madagascar, the Democratic Republic of Congo, and Peru. The Third Pandemic, which began in the nineteenth century, established new plague foci on every inhabited continent, and the disease continues to cause outbreaks in areas where rat and flea populations are not effectively controlled.