Key point: Up to 25 million people perished as a result of the plague. This is why pandemics matter but also why everyone today should keep calm and carry on.

More than six centuries ago, disaster struck the people of Europe. A deadly plague, traveling west along trade routes from Central Asia, struck the continent with such force it wiped out entire villages and killed as many as twenty-five million people. The “Black Death,” as it was called, not only depopulated Europe but set the stage for profound societal change.

The disease that was later called the “Black Death” is thought to have originated on the steppes of Central Asia, gradually brought westward along trade routes. The first appearance of the plague in Europe was at Genoa in October 1347. One hypothesis is that Italian traders caught the plague during the Mongol siege of the Crimean city of Caffa, where the attackers allegedly hurled the bodies of plague victims over the city walls. The traders fled the city, returning to Genoa with the disease. Within months, 60 percent of the city population was dead.

The Italian writer Giovanni Boccaccio lived through the first wave of plague that swept through nearby Florence in 1348. The city made extensive preparations to avoid the disease, including refusing to let the plague-stricken enter the city. Regardless, the disease manifested itself that spring, almost certainly due to the warmer weather, and increasing rat and flea activity.

Boccaccio described a world where ignorance about the plague and how to combat it spread death and paranoia. People thought that merely touching the clothing of the deceased was enough to contract plague, and shunned contact with even friends and family to avoid even the chance of contracting it. City dwellers walked through the streets sniffing perfumes to avoid the smell of the dead and the dying. The plague killed the infected so fast they died in the streets, while other died at home, unnoticed, until the smell of their decaying corpses alerted their neighbors.

From Italy the plague swept across Europe, replicating the tragedy of Genoa over and over again. The plague crossed the continent in waves and from multiple points of entry, not just Genoa, but typically through trading routes. By August 1348, it had reached southern England, and by 1350 had breached Scandinavia. By 1353, it had reached Moscow. Overall, the Black Death is thought to have killed one-third of Europe’s people, or twenty-five million people. In England, it killed half the population.

What was the plague? Scientists believe it was the bubonic plague, also known as the bacterium Yersinia pestis. Yersinia pestis typically infects the Oriental rat flea, which in turn infects small rodents such as mice, rodents and squirrels. As their rodent hosts die, infected fleas seek and bite humans. Alternately, bubonic plague can be transferred from human to human via bacterium in the infected person’s cough, although this is rare and requires extremely close contact.

A person infected with plague develops symptoms within two to six days, while a person exposed via cough can develop it within one to three days. The mortality rate for plague in the United States before antibiotic treatments were discovered was approximately 66 percent. There is no vaccine.

According to the Centers for Disease Control and Prevention, “Patients develop sudden onset of fever, headache, chills, and weakness and one or more swollen, tender and painful lymph nodes (called buboes). This form is usually the result of an infected flea bite. The bacteria multiply in the lymph node closest to where the bacteria entered the human body. If the patient is not treated with appropriate antibiotics, the bacteria can spread to other parts of the body.” Left untreated, bubonic plague can turn into septicemic plague, as the plague bacteria multiply and cause fever, chills, extreme weakness, abdominal pain, shock, internal bleeding and organ death.

A lack of medical knowledge, or even basic scientific awareness, helped spread the plague. The people of the Middle Ages certainly did not believe or even conceive of microscopic organisms capable of causing disease in human beings. As a result the entire epidemiological chain, from Yersinia pestis to Oriental rat fleas to rats and finally to humans, was incomprehensible. Humans instead blamed other sources, including miasma (bad air), foreigners, minority groups such as Jews and gypsies, and society’s general wickedness being punished by God.

The plague profoundly altered society. Society buckled as people at all levels, from nobles to peasants, died in large numbers. Renters died and were not replaced, weakening the power of the landed gentry. Peasant revolts took place in England, France, Belgium and Italy. Entire villages were wiped out. In many places, the plague killed healthy adults in largely agrarian communities, creating a shortage of food. People in plague-stricken zones avoided others, weakening the socioeconomic fabric of villages and communities.

Ironically, the plague did have some benefit. Plague survivors enjoyed a higher standard of living, due to a sudden overabundance of land and goods. Rigid societies became more flexible as deaths at the top encouraged upward mobility. A attitude of questioning authority and existing dogmas came into being, as a result of the church and state’s inability to contain the outbreaks. The plague is even credited for changing attitudes towards life and death, with the result that the wealthy became patrons of artists, writers and architects—the foundation of the Renaissance.

Outbreaks of the plague continued for the next three hundred years, including the Great Plague of London in 1665, which killed a quarter of the city’s population. Yet as widespread and deadly as it was, the plague never became a permanent resident of Europe. This and other factors, such as the unusual speed at which it spread and the lack of recorded rat die-offs, suggest to some scientists an Ebola-like hemorrhagic disease was actually responsible.

The Black Death was a tremendous tragedy for Europe, but it was also the impetus for societal upheaval. The Europe that emerged was traumatized but more dynamic than ever, set on a slow path of philosophical, scientific and geographic discovery that eventually spread worldwide. In a way, the survivors of the plague inoculated humanity against the future outbreaks of disease through the spread of science.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This first appeared in March 2017.

Image: Reuters