Bubonic plague

• 1. Introduction

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  Bubonic plague killed an estimated 50 million people in the 14th century alone, including somewhere between 25% and 60% of every living European. The word 'bubonic' derives from the Greek word for groin, a hint toward where the disease leaves its most recognizable mark: a swollen, tender lump that grows more painful by the day. Without treatment, the bacterium responsible kills between 30% and 90% of those it infects, usually within ten days. What actually transmits it? Why did it return across centuries in wave after wave? And what does it take to stop it?

• 2. The Flea and the Rat

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  Xenopsylla cheopis, the Oriental rat flea, is the most effective carrier of Yersinia pestis ever identified. The bacterium forms thick aggregates in the flea's gut, creating a biofilm that blocks the foregut. When the flea tries to feed on a new host, it cannot swallow and instead regurgitates infected blood directly into the bite wound. The bacteria then travel through the lymphatic system to the nearest lymph node, where they colonize and multiply. Y. pestis has an unusual defense: it resists being consumed by the body's immune cells, called phagocytes. It can reproduce inside them and kill them.

  Rats were the ideal amplifying host. Their blood can accommodate high concentrations of the bacteria without the animal dying quickly, allowing infected rat populations to sustain an outbreak for extended periods. The disease typically jumps to humans only when the local rat population collapses from mass infection, leaving infected fleas with no rodent hosts to feed on. Other flea species, including Pulex irritans and Ceratophyllus fasciatus, can carry the disease, but Xenopsylla cheopis was the dominant transmitter. Black rats, which prefer grain, traveled aboard bulk grain fleets from Africa and Alexandria to heavily populated port cities, and the manual unloading of those ships put dock workers in direct contact with flea-infested cargo.

• 3. What the Body Endures

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  Two to seven days after a flea bite, the infected lymph nodes begin to hemorrhage, killing surrounding tissue. The buboes grow most often in the groin, armpits, or neck, near the initial site of infection. They increase in size and pain over time, frequently to the point of rupturing. Acral necrosis, the dark discoloration of tissue at the extremities, can affect the nose, lips, toes, and fingertips. One documented case shows a person recovering from disseminated plague with necrosis across the nose, lips, and fingers, and bruising across both forearms.

  Advanced infection brings additional symptoms: continuous vomiting of blood, known as hematemesis; spleen inflammation; lenticulae, the black dots scattered across the body's surface; delirium; coma; and organ failure caused by bacteria entering the bloodstream. Bubonic plague can escalate to septicemic plague when bacteria spread through the blood, and to pneumonic plague when they reach the lungs. Pneumonic plague can then spread person to person through the air, without any flea involved. Treated early with antibiotics including streptomycin, gentamicin, doxycycline, or ciprofloxacin, the mortality rate falls to roughly 1% to 15%. Streptomycin given within 12 hours of infection has proven dramatically effective.

• 4. Three Pandemics

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  The Plague of Justinian began in 541 AD, striking both the Sasanian Empire and their rivals in the Eastern Roman Empire simultaneously. Emperor Justinian I contracted the disease and survived through extensive treatment. His historian Procopius described his personal encounter with the outbreak in Volume II of History of the Wars. In the spring of 542, the plague reached Constantinople, spreading from port city to port city around the Mediterranean and then moving inland into Asia Minor, Greece, and Italy. In his Secret History, Procopius declared Justinian a demon who had either created the plague or earned it through his sinfulness. The pandemic is estimated to have killed 25 million people in the 6th century alone, with recurrences over two centuries pushing total deaths toward 50 million.

  The Black Death of 1346-1352 claimed one-third of the European population, with some regions seeing mortality rates of 70% to 80%. Arab historians Ibn Al-Wardni and Almaqrizi traced its origin to Mongolia; Chinese records show a large outbreak there in the early 1330s. In 2022, researchers presented evidence pointing to the region near Lake Issyk-Kul in Kyrgyzstan as the geographic origin. The European outbreak likely began at Caffa, a trading post in Crimea, where the Mongol commander Jani Beg in 1343 catapulted plague-infested corpses over the city walls, one of the earliest documented uses of biological warfare. Italian merchants fled by ship, carrying infected fleas and rats across the Mediterranean.

  The third pandemic began in Yunnan Province, China, and had killed 80,000 people in Canton alone by June 1894. Daily water traffic carried it to Hong Kong, where over 2,400 died within two months. Shipping routes then spread it globally. The plague infected residents of San Francisco's Chinatown from 1900 to 1904, and again in Oakland and the East Bay from 1907 to 1909. The World Health Organization declared the pandemic effectively over in 1959, when worldwide annual deaths dropped below 200.

• 5. Isolating the Cause

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  During the 1894 Hong Kong outbreak, Japanese bacteriologist Kitasato Shibasaburo isolated a bacterium from plague victims just days before the Swiss-French physician Alexandre Yersin did the same. Kitasato's description was imprecise, and he expressed doubts about whether his isolate was actually responsible for the disease. Yersin's identification was clear and conclusive. The bacterium carries only Yersin's name: Yersinia pestis.

  Modern archaeo-molecular research has extended the bacterium's traceable history back thousands of years. Ancient DNA extracted from the teeth of human remains has identified Y. pestis variants moving across Asia and Europe between roughly 2,800 and 5,000 years ago. The bacterium has been found in archaeological material from the Late Bronze Age, approximately 3,800 years before the present. In 2013, English archaeologists uncovered a burial mound containing 17 bodies, mainly children. Radiocarbon dating placed them in the 1530s, and dental analysis confirmed the presence of Yersinia pestis. Some researchers have proposed that even earlier plague may have contributed to the Neolithic population decline, long before any written record of the disease.

• 6. Where It Persists Today

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  Between 2010 and 2015, there were 3,248 documented plague cases worldwide, resulting in 584 deaths. The Democratic Republic of the Congo, Madagascar, and Peru report the greatest number of cases. The Democratic Republic of the Congo alone recorded more than 1,100 cases in the decade following 2001. From 2012 to 2017, Madagascar hosted regular epidemics tied to political unrest and poor sanitation. In November 2017, the Malagasy Ministry of Health reported an outbreak larger than any recent one in the country, with most cases unusually taking the pneumonic rather than bubonic form.

  In the United States, 1,036 human plague cases were recorded between 1900 and 2015, averaging 9 per year. In 2015-16 people in the western United States developed plague, including 2 cases in Yosemite National Park. Cases concentrate in rural northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon, and far western Nevada. In May 2019, a couple died in Mongolia after hunting marmots. In July 2020, a teenager in Mongolia died after consuming infected marmot meat, and officials in Bayannur, Inner Mongolia, activated a city-wide plague-prevention system for the rest of that year. Current computer modeling suggests the bacterium did not disappear between pandemics but persisted within rat populations at subclinical levels, an insight that helps explain why the disease has never truly vanished.