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What Caused the Black Death?

The Black Death in Medieval Europe

A later medieval (c 1491) altar piece depicting the intercession of St. Sebastian with God during the Black Death

A later medieval (c 1491) altar piece depicting the intercession of St. Sebastian with God during the Black Death

The Black Death: Medieval Plague

In the middle of the fourteenth century a plague emerged in the East. It rapidly spread west, tearing through Europe with alarming speed. People were unable to resist it; without understanding its cause, they were unable to outwit it. Many attempted to outrun it, but it ruthlessly followed, cutting down around a third of the population. We have come to know this plague as the Black Death.

The people of medieval Europe had no way of knowing what was killing them; it wasn't until the late nineteenth century that scientists began to find an answer and it's only in the last few years that scientists believe that they categorically know what caused the Black Death.

The Black Death Reached England in 1348

A present day plaque in Weymouth, UK, commemorating the place where the Black Death entered England.

A present day plaque in Weymouth, UK, commemorating the place where the Black Death entered England.

Medieval Views on the Causes of the Black Death

At the time of the Black Death medical science was almost non existent. People did want answers and a cure but there were precious few who were qualified to give advice. The King of France received a report from his medical advisers in Paris that placed the blame on an ill-favoured conjunction of three planets. This, they assured the King, had resulted in a great pestilence in the atmosphere which was causing the plague.

Most people did assume that the plague was airborne and many people attempted to ward it off by sweetening the air with herbs. Not everyone believed that the bad air was caused by the planets; some attributed it to fumes released by earthquakes, others to strange weather and there were further theories that the corrupt air arose from stagnant water.

More sinister were the accusations against the Jews. In Savoy, some Jews were tortured and unsurprisingly confessed to a conspiracy to poison Christians, suggested by their interrogators. News of the "conspiracy" spread from Savoy through Switzerland and into Germany. Thousands of Jews were burnt in an attempt to stop the plague. Of course, the Black Death wasn't halted. It continued to spread, reaching England where at least the Jews couldn't be blamed; they had all been expelled from the country some sixty years previously.

As the plague dragged on and more and more people died, a view emerged that the plague was God's punishment for sin. The Church may have encouraged this idea as it encouraged people into the churches. Any pleasure they might have felt at being able to lead the fight against the plague must have been short lived. Prayer did not defeat the Black Death and by encouraging their parishioners to believe that it would, priests simply disillusioned them.

The Bacterium that Caused the Black Death

Yersinia pestis, the bacterium responsible for the Black Death

Yersinia pestis, the bacterium responsible for the Black Death

Yersinia pestis, the Bacterium that Caused the Plague

Yersinia pestis is a bacterium that causes three forms of plague.

  • Bubonic
  • Pneumonic
  • Septicemic

Infected Fleas Transmitted the Black Death

The black area is the Yersinia pestis bacterium blocking the flea's gut; this would be regurgitated into a new host.

The black area is the Yersinia pestis bacterium blocking the flea's gut; this would be regurgitated into a new host.

The Modern View of the Causes of the Black Death

It wasn't until the very end of the nineteenth century that two scientists finally unravelled the causes of the Black Death. Plague broke out again in China in 1865 and in 1894 scientists began researching the outbreak. Alexandre Yersin, a French/Swiss physician, was one of the team that isolated the bacterium responsible for the epidemic and which now bears his name: Yersinia pestis.

Yersinia pestis is only part of the cause of the plague. Several years after Yersin identified the bacterium, another scientist, Paul-Louis Simond worked out how the bacterium was spread. Simond surmised that the plague was spread by fleas, in particular the Oriental rat flea (Xenopsylla cheopis). He noted that the guts of fleas that fed on an infected host were blocked by the bacterium. The fleas then went into a feeding frenzy, during which they attempted to clear the blockage by regurgitating it when they bit a new host. This then infected the new host with the bacterium.

Along with the bacterium and the fleas, there was another group of unwitting villains needed to facilitate the Black Death: rodents. Not just any rodents, but two types. One group had immunity to the disease and it was these rats that were able to carry the infected fleas. A second group lacked resistance and once these rats died, the fleas left them and spread onto other hosts (including people) and infected them with the plague.

Finally, plague spread so widely because of growing medieval trade. There was a brisk trade between East and West for goods and its was the transport of these goods along the Silk Road and across the Mediterranean that allowed the plague to sweep across Europe. Hidden amongst the silk and spices would have been a deadly cargo: flea ridden rats.

Modern Day Plague

Yersinia pestis still causes thousands of cases of plague each year. Around 10-20 cases are reported in the USA annually, most frequently occurring in the south-western states. Generally, the victims' outlook is good, due to treatment with antibiotics.

More Information About the Black Death

There were many reported symptoms of the Black Death from headaches to pus-filled swellings. Find out more about the signs of the Black Death.

Doubts About the Causes of the Black Death

Simond's idea of the Black Death being spread by fleas via rats was accepted until the 1970s. However, several modern researchers have challenged his findings. Doubts have been expressed because of:

  • the lack of reliable statistical evidence of the final death toll, leading some to believe it was exaggerated;
  • the belief that fleas could not survive in Northern Europe's cold winters;
  • the view that the rat population would not have been large enough to have transmitted the disease so widely;
  • doubts that sufficient rats would have been transported with goods to make a significant impact;
  • differences in the symptoms of modern bubonic plague outbreaks and symptoms described for the Black Death;
  • differences in the patterns of transmission between modern outbreaks and the medieval pandemic.
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In 2010 and 2011 further studies were made of remains recovered from plague pits in Europe. The research concluded that a branch of the Yersinia pestis bacterium was responsible for the Black Death, although this variant, an ancestor to modern strains, is now extinct. It was further concluded that there hadn't been one wave of the Black Death, but at least two and possibly more, each perhaps caused by a slightly different strain of the older Yersinia pestis bacterium.


Judi Brown (author) from UK on September 23, 2012:

Thanks Nick - amended the paragraph. Thanks for taking the time to comment.

Nick on September 23, 2012:

Just one comment - the name: People of 14th C Europe did not use the term "Black Death", but "The Great Pestilence", "Great Plague" or " Great Mortality". The term "black" was first used in the 16th C by Danish & Swedish chronicles, and it was used to describe the sense of terror or dread that occurred. The actual term "Black Death" was coined in 1833

Judi Brown (author) from UK on June 08, 2012:

Hi Just History - ha, we had that problem (fleas, not plague) this winter. Mild winter, fleas not killed off, flea-ridden dog in no time.

Thanks very much for taking the time to comment, always appreciated :-)

Just History from England on June 08, 2012:

Any one who has had their pets bring in fleas will know that they spread and grow so quickly and are really difficult to eradicate- No wonder the plague spread

Judi Brown (author) from UK on June 05, 2012:

Thanks joanveronica - will take your advice!

Joan Veronica Robertson from Concepcion, Chile on June 05, 2012:

Hey Judi, if writing more about the plague, etc., why don't you put them into a group? It helps views a lot, and also helps us interested readers. See you!

Judi Brown (author) from UK on June 05, 2012:

Hi joanveronica - yes, I'm on a roll with the Black Death! Got another planned too, hope you will enjoy that too, when I publish it.

Thanks once again, I appreciate your comments :-)

Joan Veronica Robertson from Concepcion, Chile on June 05, 2012:

Hi, good Hub! I read it with great interest, having already read a later one. Congratulations, well written. Voted up, awesome and interesting.

Judi Brown (author) from UK on June 05, 2012:

Hi scarytaff - yes, the plague kept popping up from the 7th century right through until around the 17th century. What made this outbreak worse seems to have been the unhappy mixture of all three forms of the plague, possibly with other diseases mixed in. I've got a few Black Death ones of my own lined up, but I am off to read yours now.

Thanks for your comments, much appreciated :-)

Judi Brown (author) from UK on June 05, 2012:

Hi classicgeek - thanks for adding some more information, that's very interesting! I've not seen any plague columns, but one of the churches in Bristol has a room stacked with skulls of plague victims - quite scared me as a child.

Thanks for taking the time to comment, much appreciated :-)

Derek James from South Wales on June 04, 2012:

Good hub, Judi Bee. You're right, the rats carried the fleas and the plague swept across Europe many times in this century. My hub, supports you.

classicalgeek on June 04, 2012:

One person who actually had some useful advice in treating plague patients is someone you have probably heard of but never thought about in conjunction with medicine: Nostradamus! He believed in cleanliness and fresh air, and many of his patients survived the Black Death! (Incidentally, one of the reasons that the plague did not spread earlier is that early medieval people loved to take baths. It is only towards the Renaissance that the belief was spread that baths were bad for you. Since fleas drown immediately when submerged in water, this makes a lot of sense on the timing!)

I have been to visit many plague columns in Europe; it's quite a sobering experience to think on.

Judi Brown (author) from UK on June 04, 2012:

Hi UnnamedHarald - the Black Death was definitely a world-altering event with far reaching consequences. As you say, many of the survivors found that their lot in life actually improved. Wouldn't recommend plague as a vehicle for social change though! Planning a couple more on this theme, so stayed tuned!

Thanks for your comments, appreciated as ever :-)

David Hunt from Cedar Rapids, Iowa on June 04, 2012:

Another great article, JudiBee. The Black Death had huge and innumerable impacts on Eurape. It's impossible to imagine what life was like during its ravages. I've also read that the printing industry took off due to the glut of rags available for paper-making due to the sudden death of so many millions whose clothes were no longer needed and who no longer need to be clothed. Also, for the first time, surviving commoners found their services were at a premium because of the drop in population, leading to a slight increase in their quality of life. Voted up and interesting.

Judi Brown (author) from UK on June 04, 2012:

Hi Mhatter99 - great to hear that you found this interesting. Thanks very much for taking the time to comment, I appreciate it :-)

Martin Kloess from San Francisco on June 03, 2012:

Thank you for this fascinating report

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