The Real (and Perceived) Cause of the Plague: Part 2

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This is a continuation of the first part in a series about what caused the Black Death in 12th-17th century Europe, and part of Plague Week here at Retro, which is looking like it might become Plague Fortnight.

The first appearance of the Black Death in Europe was sometime in the 14th century, however major bouts of Plague recurred almost every generation until the 1700s when sanitation improved dramatically. Rare "modern-day" bouts of the plague still occur occasionally in livestock and people, but the invention and standardization of anti-bacterial drugs drastically improved the chance to quell the infection and survive.

In Part One, I mentioned that the main scientific consensus regarding the cause of the Black Death was a particularly nasty (and fast-acting) bacteria called Y. pestis, which is responsible for bubonic plague. However, there exist a few alternative theories that scientists have offered over the years. Specifically, that while the modern-day bouts of plague are indeed Y. pestis, the widespread deaths in the 14th century were not from bubonic plague but from anthrax or an Ebola-like virus..

(Continued below the fold...)


Several historians have noted that if what caused the Black Death was truly transmitted by fleas, as Y. pestis is, then areas which had no rats or fleas would not have had great losses due to the plague. However, about two-thirds of Iceland's population was wiped out during the 14th century, despite it having no rats at the time. And many areas with hot summers above 82 degrees F, which is too hot for fleas to transmit Y. pestis[1], were hit equally hard by the Black Death. The medieval plague and the modern plague seem to peak in different seasons--the modern plague in temperate weather and the medieval plague in quite hot weather. Symptoms also vary somewhat, as the location of the "buboes" in the medieval plague were in the lymph nodes in the groin, armpits, and neck while the modern plague buboes are mostly limited to the groin. Another issue is the rate and depth of transmission of the disease. Y. pestis has a brief incubation and high rate of fatality to the infected patient, so how did the Black Death spread so rapidly and so far? Most people would be bed-ridden in a few days, and dead in a few more. Perhaps another pathogen is to blame.


These questions led some scientists, Graham Twigg and Norman Cantor in particular, to theorize that the Black Death may have been attributed to anthrax (Bacillus anthracis) or the "cattle anthrax" murrain rather than Y. pestis. Anthrax is also spread by a bacteria and often fatal. In this case, the disease would have been spread by grass-eating livestock like cows or pigs, and people could become infected by eating meat from an anthrax-infected animal. This theory was bolstered somewhat by the discovery of anthrax spores in a plague pit (burial site for plague victims) in Scotland as well as the discovery of recorded symptoms during the Black Death not indicative of Y. pestis. Others claim that the Black Death was actually many separate contagions, with anthrax being one of them.

Ebola-like Virus

In addition, Susan Scott and Christopher Duncan believed that the Black Death might have been attributed to a virus as opposed to bacteria, and proposed an Ebola-like virus as the culprit. They claimed that this would have solved the problem as to how the plague spread far and fast, as this virus would have had a much longer incubation period therefore allowing more people to become infected in the meantime. They noted that church records from the 14th century listed sometimes 30 days incubation before death, which may have been too slow for a Y. pestis infection. In addition, they claim that a virus would have been much more efficient in human-to-human transfer and infection than a bacterial infection would have, and better explains the widespread nature and high contagion of the disease. It is interesting to note that Europeans do have an increased immunity to Ebola-like viruses, higher than the rest of the world, and this may be explained by its emergence during the Black Death.

Probably still Pestis

Despite these theories, most epidemiologists and plague historians agree that Y. pestis is still the best explanation of the widespread deaths during the Black Plague of medieval Europe. The remains of some of the victims of the plague have confirmed that Y. pestis was active during that time-- dental pulp from a plague cemetery in Montpellier, France confirmed DNA from Y. pestis and were negative for anthrax. As for the "Iceland problem," it is quite likely that European rats infected with the plague stowed away on ships bound for Iceland. In such closed quarters there was a high probability of infecting a high proportion of the crew (who then transmitted it to people in Iceland). Furthermore, infected rats survived longer than infected humans and may have lived to see Iceland, breed, and infect other mammals. As for the transmission of Y. pestis from human-to-human, this was quite possible as the bacteria became aerosolized in coughed-up water droplets which others breathed in. A ship could be the ideal place for that to occur.

So, in summary, while there are still a few mysteries to work out as to how the plague was definitively transmitted and why/how it incubated and spread, it seems safe to say that Y. pestis was indeed the cause of the Black Death.

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Close up of a 14th century talisman against the plague.

[1] Plague is transmitted while the infected flea is feeding, by regurgitation of the bacillus from the flea's alimentary tract through the proboscis into the new host. Yersinia pestis is rapidly eliminated from the proventriculus of the oriental rat flea when the mean monthly ambient temperature exceeds 28 degrees C (82 degrees F), and plague epidemics decline with the advent of hot weather (Cavanaugh, 1971).


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Hi i enjoyed your article. i used it in my project. thanks!

By Jeena Wilkerins (not verified) on 13 May 2011 #permalink

How did the plague get a name like Black Death? You had mentioned in your last post that it got this name only in the 19th century sometime, so could there racist undertones here? If this is true then it is really unfortunate that it is still being used today and nothing is being done to be politically correct about it.

From Wikipedia ( ):

... the term "Black Death" was introduced for the first time in 1833.[15] It has been popularly thought that the name came from a striking late-stage sign of the disease, in which the sufferer's skin would blacken due to subepidermal hemorrhages (purpura), and the extremities would darken with gangrene (acral necrosis). However, the term most likely refers to the sense of "black" (glum, lugubrious or dreadful).[16]

By Michael Sartain (not verified) on 11 Jan 2008 #permalink

I've been fascinated by the plague ever since reading Connie Willis' THE DOOMSDAY BOOK eons ago (closely followed by PLAGUES AND PEOPLE). Thanks for doing this most excellent series!

It may be Bacillus anthracis
(In Scotland, you said, they found traces)
Or Ebola-like virus
That tries to expire us
Its nasty, whatever the case is.

I also must say, for my part,
That I love all the skeleton art;
But as cool as I find it
The reasons behind it
Still, time and again, break my heart.

What's most irksome about Cantor's "In the Wake of the Plague" is that he seems to be writing a typical historical synthesis, but sneaks his pet theory in. "Soup-making" as my old history teacher called it. If you have a theory that challenges the established ones, by all means publish it, but do so under true colours. Even Gavin Menzies did as much with his "The Year China Discovered America" silliness.

Others claim that the Black Death was actually many separate contagions, with anthrax being one of them.

Any thoughts on a concurrent typhus plague being involved along
with the Yersinia pestis epidemic as well?

By Dark Matter (not verified) on 11 Jan 2008 #permalink

"Any thoughts on a concurrent typhus plague being involved along with the Yersinia pestis epidemic as well?"

Hans Zinsser speculated on the possibility in his Rats, Lice and History, which is a "biography" of typhus. It seems unlikely though, because the rash that commonly occurs in typhus was seldom reported in plague cases. That doesn't rule out that there was some typhus occurring in the plague period, but it probably was not a major factor.

Another observation that Scott and Duncan make that lends credence to the hemorrhagic virus theory is the fact that sites that did perform some type of quarantine for the infected reduced their death toll.

Dwellings then (and even now) were fairly permeable to rodents and their parasites, the fact that locking someone in their home reduced the transmission rate shows strong evidence of person-to-person transmission. ie quarantining someone would have had little effect if rats and fleas were spreading the disease.

ps. I'm not involved in any of the sciences (software engineer) but I found their book to be an incredibly interesting read.

Ireland might be a good case to study differential effects of the Black Death because two communities lived side-by-side in different circumstances: The medieval Eenglish colonists lived mainly in towns, concentrated in the East and South-East; the native Irish inhabited the rest of the island under a set of petty chieftains. Much like an Arab country where the State runs the cities and foreign trade, and the Bedouin continue their own way of life apart.

The English colony suffered greatly in the Plague, and the attendant depopulation is often given as one reason why the English conquest was rolled back in the 14th and 15th centuries. The Irish lived in more dispersed communites with less internal trade and maybe did not give the disease as big a chance to spread. They also farmed mainly pasturage, and spend a lot of time outdoors (in spite of the rain!).

On the other hand, the Irish way of life might have provided some sort of accidental quarantine, and lend weight to the virus theory.

A lecture about the plague a few years ago started out: "A horrible disease struck Europe in the 12th Century. Bodies littered the streets. Thousands died. Millions died. Billions died.... The flea population of Europe was decimated."

Hi Shelly,

This minimally qualifies as 'plague' trivia.

CSI New York [CBS] on 1/9/08 aired 'Episode 412: Happily Never After'.
There was reference ro the poem 'Ashes, ashes, all fall down' even though the episode had nothing to do with the black plague.

Variations of the poem 'Ashes, ashes, all fall down' appears on this page from Division of the Arts, UC-Santa Cruz.

People were much tougher back then, because of the lack of things like antibiotics, So PESTIS could have had a longer incubation period and somewhat different symptoms in a stronger population of vectors.
Also, has Pestis itself mutated in these centuries. Has anyone done a genetic comparison on the strains found in old graves of Plague Victims and modern Pestis?

People were much tougher back then, because of the lack of things like antibiotics

Ummm... not sure what you mean by this, but I question several things it might mean :-)

Tougher because antibiotics have allowed more "less tough" genomes to survive and reporoduce? I don't think antibiotics have been around long enough to shift gene frequencies that much. Better sanitation (public and private) go back farther -- and are often credited with a bigger over-all impact on public health than modern medicine per se -- but they too are awfully recent in terms of population genetics.

There may be some tangential support what we know about e.g., polio and mumps -- two pathogens that used to be very common exposures in early childhood, with generally mild or even unnoticed symptoms. With better sanitation, people were first exposed later in life, with more serious symptoms. But that seems to hinge on a changing immune system within individual maturation, not a population effect.

Tougher in that people without antibiotics have stronger immune systems, leading to longer incubation before a pathogen "breakout" causes overt symptoms? Is there evidence for that as a direct effect, rather than (again) mediated through age or frequency of exposure to pathogens?

Of course, if by "tougher" you mean "they took more pain as an inevitable part of life than we do, and so complained less per unit pain"... that seems likely. I'm a wimp compared to anybody who lived before dental anesthesia, central heating, etc., and not ashamed to asdmit it.

My own guess is that despite the intuitive appeal of 'more diseases, fewer cures = tougher,' nutrition and wider travel + interbreeding probably net out to make us more disease-resistant rather than less.

I'm finishing up my own posts on this topic, and while I was re-reading this I have to nitpick one thing: Scott and Duncan aren't epidemiologists. Scott is a social historian and demographer, and Duncan is a zoologist. One annoyance of mine is that neither of them do have any training in infectious disease epidemiology; if they had, IMO they wouldn't make statements about what microbes "can't" do as they do in their writings.

The English colony suffered greatly in the Plague, and the attendant depopulation is often given as one reason why the English conquest was rolled back in the 14th and 15th centuries. The Irish lived in more dispersed communites with less internal trade and maybe did not give the disease as big a chance to spread. They also farmed mainly pasturage, and spend a lot of time outdoors (in spite of the rain!).