Causes of the Black Death
Infectious Bacterial Disease Theory
The most supported theory explaining the cause of the Black Plague is that it is an infectious bacterial disease. This theory identifies the bacterium Yersinia pestis as the cause for the illness’s high mortality rate and lack of effective treatment. The bacteria’s spread is explained through fleas and rats, specifically, Oriental rat fleas and two populations of rodents: one resistant to the infection, the other not resistant. After the disease wipes out the second population (rats that die when infected), the fleas must find another host population: humans. Rats and fleas drove the spread of the infection within their own communities, and then transferred it to humans as they were taken along trade routes from the Plague’s starting point in central Asia westward to Europe.
http://worldhistoryforusall.sdsu.edu/eras/era5.php
This map shows the flow of the Plague in the 14th century based on the reports of outbreaks that are verifiable, giving support to this theory that the Plague began in Asia.
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http://www.vlib.us/medieval/lectures/black_death.html
Looking at Europe specifically, we can see even more movement along trade routes. Goods from Asia arrived in Italy and then took the Plague with them as they traveled radially into other parts of Europe.
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Viral Hemorrhagic Fever Theory
A theory that challenges this is that the Plague was really a viral hemorrhagic fever. This theory uses the disparity between the slow transportation speeds of the Middle Ages and the huge spread of the disease to suggest an alternative to the traditionally accepted theory of an infectious bacterial disease. A viral fever (e.g. Ebola) is characterized by a long incubation period, in this case 32 days, which would have driven the sickness by allowing each infected person or animal to contaminate many other hosts before showing significant symptoms. Another reason that this alternative theory was developed is because descriptions of the symptoms of the plague are not always clear, and to some, they are more indicative of this type of disease than bubonic plague. Supporters of the viral fever theory contend that the single symptom of swollen lymph glands (buboes) has been erroneously attributed as characteristic of bubonic plague only. This theory also proposes that the Black Death began in Africa rather than Asia and subsequently spread to Europe and eastward. Historical support for the theory of viral fever focuses mainly on the lack of rat and flea populations in certain plague outbreaks, as well as the disparity of the geography of plague symptoms and mortality compared with that of gene mutations for HIV immunity.
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This map lends support to the theory of a viral fever cause because it shows clear northward movement of the plague (from Africa) rather than westward (from Asia), but it disregards the reports of plague in places like Tibet and Mongolia prior to 1347.
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Alternative Theories
There are several alternative theories of the cause of the plague, as well as criticisms of the most popular theories. First, many researchers agree that other diseases accompanied the plague itself, such as worms, pox, fevers, and dysentery, explaining the sometimes varied reports of symptoms. The proposition of rats as significant carriers of the Plague has also been criticized because the population was not large enough in many areas to cause the amount of human death that occurred, and by the end of an outbreak, there would have been far fewer surviving rats than many records indicate. DNA results have been called into question because of results of testing particular mass grave sites have not been repeated elsewhere. The problem that is challenged in relation to mortality rates is that they are very similar in rural and urban areas, and this is not how bubonic plague behaves today. The modern evidence of bubonic plague differs from Black-Death-era reports in amount of mortality, location of severity, and timing of outbreaks. Other alternatives to the major theories are variations on the order of transmittance (e.g. rat flea- black rat- human) and the assertion of anthrax as a causative agent instead of Yersinia pestis.
Specific Causes: Bubonic, Septicemic, and Pneumonic Plague
Assuming the most widely held opinion, the Black Death was a plague in three clinical forms: bubonic, primary septicemic, and primary pneumonic.
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This visual shows that both domestic and wild rodents and fleas would pass the disease within their own communities, then come in contact with a human, giving them bubonic plague. If the human didn’t die first, the disease would progress to pneumonic plague, meaning it would be highly contagious and extremely deadly.
http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Yersinia_pestis.htm
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http://www.cdc.gov/plague/faq/
Works Cited:
"Black Death." The Cambridge Historical Dictionary of Disease. Cambridge: Cambridge University Press, 2003. Credo Reference. Web. 1 December 2013.
"Black Death." Wikipedia. Wikimedia Foundation, 30 Nov. 2013. Web. 30 Nov. 2013. <http://en.wikipedia.org/wiki/Black_Death>.
Duncan, C. J., and S. Scott. "What caused the Black Death?." Postgraduate medical journal 81.955 (2005): 315-320.
Edmonds, Molly. "How the Black Death Worked." HowStuffWorks. N.p., n.d. Web. 30 Nov. 2013. <http://history.howstuffworks.com/historical-events/black-death.htm>.
"Pnuemonic Plague." Pnuemonic Plague. N.p., n.d. Web. 01 Dec. 2013. <http://people.uwec.edu/piercech/bio/Pnuemonic Plague.htm>.
Zietz, Björn P., and Hartmut Dunkelberg. "The history of the plague and the research on the causative agent< i> Yersinia pestis</i>." International journal of hygiene and environmental health 207.2 (2004): 165-178.
"Black Death." The Cambridge Historical Dictionary of Disease. Cambridge: Cambridge University Press, 2003. Credo Reference. Web. 1 December 2013.
"Black Death." Wikipedia. Wikimedia Foundation, 30 Nov. 2013. Web. 30 Nov. 2013. <http://en.wikipedia.org/wiki/Black_Death>.
Duncan, C. J., and S. Scott. "What caused the Black Death?." Postgraduate medical journal 81.955 (2005): 315-320.
Edmonds, Molly. "How the Black Death Worked." HowStuffWorks. N.p., n.d. Web. 30 Nov. 2013. <http://history.howstuffworks.com/historical-events/black-death.htm>.
"Pnuemonic Plague." Pnuemonic Plague. N.p., n.d. Web. 01 Dec. 2013. <http://people.uwec.edu/piercech/bio/Pnuemonic Plague.htm>.
Zietz, Björn P., and Hartmut Dunkelberg. "The history of the plague and the research on the causative agent< i> Yersinia pestis</i>." International journal of hygiene and environmental health 207.2 (2004): 165-178.
Header Picture Cited:
http://www.2020site.org/history/bubonic-plague-in-the-middle-ages.html
http://www.2020site.org/history/bubonic-plague-in-the-middle-ages.html