How the Black Death Worked

Black Death in European town
The Black Death in a European town Hulton Archive/Getty Images

In his introduction to "The Decameron" (circa 1351), Italian author Giovanni Boccaccio captures the uncertainty and fear that accompanied the Black Death's sweep through Europe from 1347 to 1351. It could have been caused by the influence of celestial bodies or by God's wrath, Boccaccio wrote. Physicians had no idea what to do with the tumors and black spots that ravaged victims' bodies. And worse, the people turned on each other. Parents abandoned children; husbands turned their backs on their wives. The streets were filled with the dead, and neighbors sometimes learned of a death next door by the smell. The living didn't have time to mourn; Boccaccio says that due to the large number of dead and the fear of their presence, "a dead man was then of no more account than a dead goat would be today."

The symptoms of the Black Death were gruesome: Tumors covered the body -- some of them as big as an egg or apple, Boccaccio wrote. A large neck tumor might permanently cock a person's head in the opposite direction. Purplish splotches also covered the body. These were nicknamed "God's tokens," because God usually took the sufferer soon after they appeared. The sick even smelled like they were going to die. Bad breath and odors indicated they were rotting from the inside.


Medieval writers tell us that the fevers resulted in delirium -- madmen wandered the streets, shouting wildly. The sick vomited incessantly or coughed up blood. Pus and blood oozed from sores. Once the symptoms started to appear, the victim was a ticking time bomb and died within days. No one knew what to do. There wasn't enough space in the graveyards, so the bloated bodies were left in the street. Dogs ate corpses while babies cried hungrily beside their dead mothers.

When all was said and done, about 25 million people died in the epidemic, approximately one-third of Europe's population at the time [source: Kelly]. How did this happen? What caused this horrific spread of death?

How Did the Black Death Spread?

Black Death spread illustration
The Black Death moved through Europe astonishingly fast. Take a look at some of the stops on its path.

Because Europe was trading with the East, some medieval Europeans were aware of a mysterious disease sweeping through Asia in the 1330s. From Central Asia, the disease moved along an established trade route, passing through Turkestan and the Black Sea Region (Crimea and the Byzantine Empire).

In 1347, Kaffa, a town in modern-day Ukraine that was a Genoese trading post, came under attack by a Tartar army. When the Tartars were killed by the plague, the Genoese at first rejoiced: God had answered their prayers and punished their enemy. But that celebration ended when the Tartars began launching the corpses of plague victims over the walls of the city, hoping that the smell of rot would kill everyone in town. The smell didn't kill the Genoese, of course, but the disease did. The panicked Genoese threw the corpses back or submerged them in water. But it was no use; they were already exposed. As the dying Tartars retreated, the Genoese fled by ship to Sicily, taking the deadly disease with them to Europe.


Kaffa wasn't the only eastern trading port on the Black Death's path, but Genoa's ships took the blame for bringing the pestilence. Once it hit Europe, the Black Death moved fast, traveling at an average speed of 2.5 miles per day (4 kilometers per day) [source: Duncan, Scott]. From the Mediterranean ports, the disease took two paths; one through France that eventually made its way to England and Ireland, and one through Italy that went to Austria and Germany.

Written accounts state that the disease was frightfully contagious, and that death occurred only a few days after symptoms appeared. Other than this, people seemed to have no idea what was happening. Many felt that God's wrath was ravaging the earth and that the end of the world was near. Some theorized that Jews were contaminating the water supply. Both of these ideas spurred extreme responses that we'll explore in the next section.

When people began dying in France, King Philip VI turned to the Paris College of Physicians, the most highly-regarded medical authorities of the time, to learn the cause. The physicians produced a report that blamed the mass deaths on an event that occurred at 1 p.m. on March 20, 1345 -- the triple conjunction of the planets Saturn, Jupiter and Mars in Aquarius. The report explained that Jupiter, a wet and hot planet, soaked up evil vapors from Earth. And Mars, a dry planet, ignited the vapors and spread them through the air, which is how Europe got enveloped in a fog of death.

Interesting -- a fog of death. So, how do you cure a fog of death? And how do you protect yourself from catching it? In the next section, we'll learn how people dealt with the ugly spread of the Black Death.

Reaction to the Black Death

Procession of the flagellants
Procession of the flagellants Imagno/Hulton Archive/Getty Images

Because the physicians blamed the Black Death on an evil, polluted fog, logical recommendations to prevent the fever involved avoiding these miasmas, or corruptions of air.

Fires were a popular method of warding off miasmas. They were burned at street corners; even the pope sat between two large fires. People were urged to burn aromatic woods, but other scents would do as well, including rosemary, amber, musk and fragrant flowers. When they walked, people took their scents with them, carrying packets of herbs. Some plague-proofed their homes by putting glazes over the southern windows to block the polluted southern wind. People were advised not to eat meat or figs and to avoid activities that would open the pores to a miasma, including bathing, exercising and physical intimacy. Stranger recommendations circulated as well, including not sleeping during the daytime and avoiding sad thoughts about death and disease.


Many medieval tracts address how to avoid sickness, but we know very little about how medieval doctors tried to cure the disease. It's possible they believed nothing could be done. Most medieval cures involved bloodletting, which was an attempt to draw poison out of the body. And we know some physicians tried to rupture and drain the buboes.

But many people instead turned to the church for a cure, praying that God would end the great pestilence. Religious reactions took two extreme forms: the rise of the flagellants and the persecution of Jews.

The Brotherhood of the Flagellants had appeared earlier in Europe, but rose up in great numbers in Germany in late 1348. They believed the Black Death was the punishment of God and took it upon themselves to try to appease him. The Flagellants marched barefoot throughout Europe, whipping themselves with scourges, or sticks with spiked tails. Enormous crowds gathered to watch the ritual beatings, complete with hymns and prayers for God's forgiveness. The pope was initially tolerant of the movement, but he denounced them in 1349, and the Flagellants disappeared, seemingly overnight.

The Flagellants were also extremely anti-Semitic, but they weren't the only ones. While anti-Semitism was already on the rise in Europe, it reached a fever pitch when many came to believe that Jews were poisoning the wells and causing the Black Death. Because Jews at this time usually lived apart from Christians in separate quarters, they were in effect already quarantined when the Black Death hit the towns, so they had high survival rates.

Vengeful Christians burned Jews at the stake or set buildings filled with entire communities on fire. Some Jews responded by setting their own homes on fire before the angry mobs did it for them. Others converted to Christianity on the spot to save themselves. While the attacks on Jews were widespread throughout Europe, some of the highest casualties were in Germany. Few Jews were left in that country by the time the plague ended.

How did Europeans emerge from this insanity and overwhelming death? Find out how their religious faith faltered and what finally fueled gluttonous celebration on the next page.

Effects of the Black Death

The Black Death reared its head sporadically in Europe over the next few centuries. But by 1352, it had essentially loosened its grip. Europe's population had been hard hit, which had an economic impact. The workforce had been destroyed -- farms were abandoned and buildings crumbled. The price of labor skyrocketed in the face of worker shortage, and the cost of goods rose. The price of food, though, didn't go up, perhaps because the population had declined so much.

The Black Death did set the stage for more modern medicine and spurred changes in public health and hospital management. Frustrated with Black Death diagnoses that revolved around astrology and superstition, educators began placing greater emphasis on clinical medicine, based on physical science. While schools initially had to close for lack of educators, the plague eventually drove growth in higher education. New schools were established, sometimes specifically mentioning in their charters that they were trying to address the decay in learning and gaps in education left by the Black Death.


People who survived the Black Death era generally suffered a communal crisis of faith. Rather than becoming more religious in thanksgiving to God for their survival, people harbored doubts. They had turned to the church for an answer to the plague, and the church had been able to offer no help. Additionally, priests, who, along with doctors, had the highest rate of contact with the diseased, also had one of the highest rates of fatalities. Several new heretical movements sprang up. Those who still clung to their faith were more likely to do so in a very personal manner. Many began to build private chapels.

Feeling, essentially, that God had turned his back on them, the people reacted to the end of the Black Death by turning their backs on him. They engaged in wild debauchery to celebrate being alive. They held gluttonous banquets, drank, wore extravagant clothing and gambled. It was clear through the art of the time, though, that people still had death on their minds. The danse macabre, or dance of death, is an allegorical concept that was expressed in drama, poetry, music and visual art. The danse macabre usually shows a procession or dance between the living and the dead. The range of figures shown is meant to show that death will come for everyone, and the various activities depicted are a reminder that death could always be right around the corner.

Townspeople flee the Black Death under the watchful gaze of skeletons
Townspeople flee under the watchful gaze of skeletons
Time & Life Pictures/Getty Images

Geneticists are continuing to document the effects of the Black Death on Europe's population today. Analysis has shown that genetic diversity in England is much lower than it was in the eleventh century, perhaps because so many people died in the 1300s. While the rest of Europe does not show a lowered amount of genetic variation, that may be due to increased migration patterns in other places.

The work of 19th-century scientists finally provided some answers about the causes of the Black Death.

Causes of the Black Death

In 1894, during an outbreak of disease in Hong Kong and India known as the Third Pandemic, bacteriologists Alexander Yersin and Shibasaburo Kitasato, working independently of each other, identified the bacteria that caused plague. This bacterium came to be called Yersinia pestis, when Yersin showed it to be the causative agent of the plague in India. Working backwards, Yersin determined that plague was the cause of the Black Death as well, due to the medieval records of large tumors.

Yersinia pestis is usually transmitted from rodent to flea to rodent. Humans are normally only targeted by fleas when there are no rodents left. When a flea bites a healthy rodent, the blood from the rodent goes directly to the flea's stomach, easing hunger. But when a flea bites a rodent infected with Y. pestis, the bacterium-riddled blood gets stuck in the flea's foregut. The bacteria will grow, engorging the flea. The flea constantly feels hungry because nothing is getting to its stomach. In response to hunger pangs, the flea feeds greedily on more rodents. It spreads the disease by regurgitating the infected blood into healthy rodents. When the rats start to die off, fleas swarm the remaining rodents. Finally, when all the rats have died, the fleas turn to people.


Was the Black Death a Virus?

Skeleton from victim of black death
A skeleton and victim of the Black Death, unearthed during London Crossrail excavations, is displayed at the London Charterhouse on Jan. 27, 2017, in London, England. Jack Taylor/Getty Images

Textbooks tell us that the bubonic plague caused the Black Death. But not everyone is convinced. Since 1984, scientists have put forward alternative explanations for the Black Death. For example, sociologist Susan Scott and biologist Christopher J. Duncan claim that a hemorrhagic fever, similar to the Ebola virus, caused the Black Death. And others blame anthrax or say that some now-extinct disease was the culprit.

Bubonic plague just doesn't make sense, they argue. The symptoms, the high mortality rate, the speed at which the disease spread, and the way the disease spread -- none of it jibes with typical bubonic plague.


Medieval accounts of symptoms don't match the symptoms of modern-day bubonic plague, either. Accounts describe buboes covering the entire body. But today, buboes would most commonly show up in the groin area, and aren't likely to spread all over the body. Additionally, medieval accounts mention awful odors, bruise-like splotches and disrupted nervous systems that resulted in delirium and stupor -- none of this happens with modern-day bubonic plague.

If the Black Death was caused by the bubonic plague, then the mortality rate was much higher than it should have been, they argue. The bubonic plague is fairly curable; even untreated, bubonic plague has a mortality rate of about 60 percent [source: Kelly]. If mostly everyone affected died, some feel that a hemorrhagic fever, with no cure, was the more likely culprit.

Proponents of these new theories also point out that bubonic plague usually moves very slowly. But the Black Death swept across Europe at enormous speed, especially given the fact that transportation was pretty undeveloped at the time. A hemorrhagic fever, in comparison, has a longer incubation period, in which people are contagious, but not yet symptomatic. People might have spent that incubation period traveling, inadvertently spreading the fever more rapidly. Writings from the Black Death also indicate that people were extremely contagious, so much so that people were scared to be in the same town as the infected. But modern-day plague outbreaks are nowhere near as contagious.

Virus advocates find other problems with the rat-and-flea bacterial infection theory. Since fleas only attack humans after all rat hosts have died, then there should have been a large die-off of rats before the Black Death. There's no evidence for a rat disappearance. Additionally, fleas require high temperatures and humidity to survive, which means that the plague should have essentially died out in winter months. It did not.

None of this reasoning has won over the scientific community yet. It's difficult to truly know what the Black Death was like. The only evidence we have are the written accounts of the time, and these accounts provide few details. Obviously, the people who wrote them didn't use our technical language for diagnosing and describing diseases. What they described as a tumor may not have been a tumor at all, by our modern-day medical standards.

To learn more about the Black Death and its aftermath, take a look at the links that follow.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Barris, Colin. "Black Death casts a genetic shadow over England." New Scientist, August 2007. (Feb. 4, 2008)
  • "Black Death." Encyclopedia Britannica. 2008. Encyclopedia Brittanica Online Library Edition. (Feb. 4, 2008)
  • Boccaccio, Giovanni. The Decameron. Trans. J.M. Rigg. (Feb. 4, 2008)
  • Cohn, S.K. and L.T. Weaver. "The Black Death and AIDS: CCR5-