Sometime in 1347 a sailing ship moored in a Mediterranean port unwittingly unleashed one of the most destructive pathogens in history. Unloaded with its cargo and passengers were some deadly stowaways: flea-ridden black rats carrying the bubonic plague. It was a scenario played out many times in ports all around Europe, and the results were always the same: Sickness, suffering, and death on what seemed a cataclysmic scale. The years 1347-1351 saw Europe in the terrifying grip of the worst pandemic it had ever suffered: At least one-third of Europe’s population died from what became known as the Black Death.
Most historians agree that it was bubonic plague, a bacterial disease that periodically flared up in Asia and Europe. The so-called Plague of Justinian devastated the Byzantine Empire in the sixth century, killing an estimated 25 million people. After the Black Death, it continued to strike large numbers of Europeans, most notably in London in 1665. The Third Plague Pandemic, the world's last major outbreak, began in the mid-19th century and lasted well into the 20th.
Medieval Europe was at the mercy of many infectious diseases, including dysentery, influenza, measles, and much feared leprosy. But it was the plague that struck the highest note of terror into people’s hearts. During its peak years, the plague spread faster, farther, and with deadlier effect than ever before or since. Its impact fundamentally altered the social, economic, and religious lives of those who survived, scarring the collective consciousness of the entire continent. It seized victims with alarming speed and its horrific ravages were incurable. None were safe as the plague cut down peasants and princes alike, its leveling of social distinctions resonating in the written accounts of the time. It is little wonder that its medieval chroniclers often assume an extravagant and even apocalyptic tone.
Many explanations of the plague were proposed, most wrapped up in religious or superstitious assumptions. Those closest to scientific reality were based on classical Greek medicine, attributing the sickness to miasmas: The invisible corruption in the air emanating from decomposing matter and supposedly absorbed by the body either by breathing or through skin contact. Some accounts suggested astrological causes, blaming the plague on the conjunction of certain planets, eclipses, or the sighting of a comet. Others cited natural phenomena: volcanic eruptions and seismic tremors releasing deadly gases. But even these explanations were widely believed to have an underlying cause: divine wrath at the sinfulness of humankind. (Here's why plague doctors wore strange beaked masks.)
Signs and symptoms
It was only during Third Plague Pandemic that the plague’s supernatural origins were definitively discarded. Researchers were able to identify the pathogen causing the disease, and in 1894 two bacteriologists—Japan’s Kitasato Shibasaburo and France’s Alexandre Yersin—simultaneously discovered the plague’s bacillus, or rod-shaped bacteria.
Later named Yersinia pestis, the bacteria was carried by fleas living as parasites on rats and other small rodents. The bacilli multiply in the gut of the flea. When it bites, it regurgitates the bacilli into the body, infecting it. Normally this takes place in a closed cycle between fleas and rodents. But under the right conditions the bacteria spreads at such a rate as to kill off its rodent hosts, forcing the fleas to find alternatives—humans. As such, the plague is a zoonosis, an illness that passes from animals to humans. Infection spread easily because the rats were drawn to human activity, especially the food supplies kept in barns, mills, and homes. (Today the plague is spreading among mountain lions in Yellowstone.)
The bacteria could be present in people’s homes for between 16 and 23 days before the first symptoms of illness emerged. Death came three to five days later. It was perhaps another week before a community became fully aware of the danger, and by that time it was too late. The nodules of a patient’s lymphatic system became infected, showing as swellings in the groin and armpit. These were accompanied by vomiting, headaches, and a very high fever that caused sufferers to shiver violently, double up with cramps, and become delirious.
The inflamed lymph gland was widely known as a bubo, giving rise to the term bubonic plague. But this was only the most common form of the Black Death—two other variants of plague were also at work. Septicemic plague infected the victim’s blood, causing visibly black patches beneath the skin, perhaps what gave the Black Death its name. Pneumonic plague affected the respiratory system, making the sufferer cough—the perfect mechanism for airborne infection. In the medieval world both septicemic and pneumonic plague had a 100 percent mortality rate. (St. Anthony's fire was another sickness that plagued medieval Europe.)
In Europe the Black Death first appeared in the Mediterranean basin and spread to most of the corners of the continent in just a few years. But the initial outbreak is thought to have been in the Black Sea port of Caffa, now Feodosiya, on the Crimean Peninsula. In 1346 Caffa was an important commercial trading post run by Genoese merchants. That year it was besieged by the Mongol army, among whose ranks were a growing number of plague sufferers.
As the disease spread, one story has it, the Mongols deliberately hurled infested corpses over the walls. Even more likely is that the bacteria entered the city in fleas carried by the rats scampering between the siege lines. However it arrived, once the city realized it faced a plague epidemic, the Genoese merchants panicked and fled, carrying the sickness with them to Italy.
Historians and scientists have puzzled about how the Black Death took such a firm hold over such a vast area in such a short time. Some have suggested that the main plague variant was pneumonic rather than bubonic because airborne transmission seems to support its rapid spread. However, pneumonic plague kills so quickly—in a few hours—that it actually spreads slowly because the host rarely lives long enough to infect many people.
Most evidence points to the Black Death being the main bubonic strain of plague, spread far and wide by flea-ridden rats on boats and fleas on the bodies and clothes of travelers. In an age of growing maritime trade, food and goods were carried ever longer distances from country to country, and the rats and their bacteria traveled with them—at an estimated 24 miles a day. The unceasing flow of sea, river, and road traffic between commercial centers spread the plague across huge distances in what is known as a “metastatic leap.” Big commercial cities were infected first, and from there the plague radiated to nearby towns and villages, from where it would spread into the countryside. The plague was also carried down the well-trodden paths of medieval pilgrims; holy sites became additional epicenters of regional, national, and international propagation.
Even without such help the plague is estimated to have moved inland more than a mile a day in the right conditions. In very cold and dry areas it slowed to a stop, explaining why Iceland and Finland were among the few places to escape its ravages. A popular refrain in cities of the time ran: “Get out soon, quick and far, and the later you return, the better.” It was advice heeded by many who could afford to flee to the countryside. Yet this brought disastrous consequences. Evacuation did not necessarily save those fleeing, as some were already infected or traveling with plague carriers. However, it did help to spread the disease to new and ever more remote places as evacuees sought the safety of uninfected villages. (Archaeologists have discovered rural mass graves of Black Death victims.)
Calculations of the fatalities caused by the Black Death are devastating and debated. Most agree that the population of Europe was estimated to be around 75 million people before the plague: It plummeted to just 50 million in the years between 1347 and 1351. Some scholars believe the fatalities could be higher.
The sharp decline was a result of both the disease itself and the widespread social breakdown it set in motion—not least that the deaths left fields and animals untended and family members uncared for. Even after the Black Death burned itself out, flare-ups continued to disrupt Europe’s demographic recovery. Not until around the 16th century did Europe’s population growth start to strengthen.
The effects of the catastrophe were apparent in every area of life. In the decades following the pandemic, wages soared because of the huge shortage of workers. Vast tracts of once productive farmland turned to pasture, and even whole villages lay abandoned—around a thousand in England alone. There was a major migration from the countryside to the cities, which recovered relatively quickly and were reinvigorated with commercial energy. The peasants who remained in the countryside were often able to take their pick of unused land, increasing the power of the landed peasantry and boosting the rural economy.
Indeed, historians have argued that the Black Death paved the way for a new wave of opportunity, creativity, and wealth from which would flourish the art, culture, and ideas of the Renaissance, and the beginnings of a recognizably modern Europe.