What cause black plague[英语论文]

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What cause black plague?
Black plague is a well known infectious disease in history for its extremely high mortality rate. This disease has changed the tunnel of our history and almost rewrote it (Norman F. Cantor). 
It was in around 1338, black plague first appeared in Central Asia, where is considered as the origin of this deadly disease. Quickly was this disease spread to India and China, and even reached the Black Sea of Kaffa by 1347. According to one famous historic record, that Kaffa was at that time surrounded by Mongols army before getting attacked by black plague. But the vicious Mongols army shot the infectious corpse into Kaffa as the final move and then the whole town of Kaffa was cast in the shadow of death. Hundreds of people died everyday and those alive were in great horror with nothing they can do to prevent this disaster. What’s worse, the devil did not linger in Kaffa,. In 1347, there were outbreaks in England, then Scotland in 1349, Russia 1351. (Norman F. Cantor) At that time, European people were horrified and desperate. They had no idea of what brought this deadly disease and how to deal with it. They tried to explain the etiology and carried out various measures to prevent or treat black plague. But all of their efforts were in vain. About two-thirds of the victims died soon after getting infected, say within three or four days of developing symptoms. Most of other s suffered about two weeks and then died in great pain. Like other infectious disease, there might be people that were the carriers of the pathogenic factor and wouldn’t present any symptoms in first few days. Typical initial symptoms of black plague included Fever, trembling, weakness, and profuse sweating. . In advanced cases, the most distinctive sign is the agonizing rise of dark "buboes": sensitive black-blue swellings under the armpit and near the groinspots
Though some places, especially those off the western coast of Scotland were unusually remained almost unaffected (Colin Platt, King), the great pandemic of black plague had killed approximately 25million Europeans from 1348 to 1352, that is, about one third to a half of the total population of European countries at that time. And the outbreaks in the following 300 years still remained great threats to survivors. It’s impossible to exactly calculate number of victims of such disease but many cities kept records of the tremendous loss. Since black plague greatly cut down the European population, it had transformed European social structure and shaked the status of Catholic Church in Rome, which dominated Europe at that time. It also gradually destructed the feudal social system and the value of workforce became important, which helped Europe walk a way to Capitalism( David Herlihy) . The plague also renewed people opinion toward public health and certain practice were carried out, which were somewhat the prototype of public health system.
But what cause this influential black plague?  The most well known and recognized theory is Yersinia pesti bateria infection through fleabites. It’s said the flea on a specific rats carried the bacteria and infected human body by their bites. Normally, the fleas that bite humans (Pullex irritans) are a separate species from the fleas that live on rats (Xenopsylla cheopis), and the bubonic bacteria can survive indefinitely in its normal host, the European black rat (Rattus rattus). Occasionally, however, a desperate flea would mistakenly bite a human host, and then the human contracts the disease. Once a human is infected, the plague bacterium can spread for a few weeks by human fleas hopping from person to person and biting them. 
After the discovering Yersinia pesti bateria as pathological factor of black plague, it became possible to treat the plague with antibiotics. Without antibiotics, the mortality rate is 72% for infected victims. However, a small number of people are naturally resistant to black plague due to unusual protein structures. The bacteria's enzymes cannot interact with these proteins easily. This protein structure seems to be tied to a specific gene. These finding might be the reason for the cities that remained intact as mentioned above. Before the 1340s, only about 0.2% of the European population appeared to have had this gene when scientists examine DNA from their remains. Now a much larger percentage of the Europeans have the gene that makes them resistant to Yersinia pestis. The 0.2% of people who were immune back in the 1300s survived the genetic bottleneck and then passed on this immunity to a significant number of their modern descendents. Today, if you are a Caucasian American, the odds are about 15% that you have inherited this gene.
However, at the early 21 centaury, modern scientist put forward a different theory of black plague etiology. They examined the bones and teeth of a number of remains and failed to find any Yersinia pesti bacteria exist by a series of high tech biological laboratory techniques. With these finding and symptoms that victims presented, these scientist overturned the old theory and established a new hypothesis that it might be a specific virus that brought the disease.  According to symptoms, DNA/RNA detection and so on, this specific virus is an Ebola-like virus and might be transmitted between human beings by saliva and blood. Ebola virus is a rare and extraordinary deadly virus that could lead to a mortality rate as high as 50%-90% ( Duncan, Scott). 
In conclusion, it’s no doubt that a microbe, rather than any religious factor, that should be blame for black plague. But the second theory that this microbe is a virus still needs further evidence as Yersinia pesti bacteria might also died and destructed after such a long period. 

Norman F. Cantor, In the Wake of the Plague . UK: HarperCollins US, 2017
Colin Platt, King, Death: The Black Death and Its Aftermath in Late Medieval England. Canada: University of Toronto Press, 1996.
David Herlihy, the Black Death and the Transformation of the West. USA: Harvard University Press,1997.

Duncan, Scott. Biology of Plagues: Evidence from Historical Populations. UK: Cambridge University Press, 2017.