The outbreak of The Great Plague (Black
Death) caused by humans


The Black Death
which occurred in the 14th Century has killed one-third of the total
population in Europe, and the bacterium (Yersinia pestis) living in rats were accused for
the spread of the disease for 700 years. However, a research that carried out
in 2017 claimed that human ectoparasites were more likely to have caused the
rapid spread of the disease than the rat flea, challenging the common concept
of spreading of the plague.



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University of Oslo and University of Ferrara’s researchers- Katharine
R. Dean, 
Krauer, Lars Walløe, Ole
Christian Lingjærde, Barbara
Bramanti, Nils
Chr. Stenseth, Boris V. Schmid reported through Proceedings
of National Academy of Sciences. BBC reported this news on the 16th
January 2018.



The researchers digitalized publicly available data from 9
localities that were affected by plague outbreaks. They
developed a Susceptible–infectious–recovered
model for the transmission of the plague. Each of the
cities was broken down further into 3 groups- Human-Ectoparasite Model (fleas
in human bodies and clothes), Pneumonic Plague Model (direct human-to-human
transmission) and Rat-Flea Model (rat fleas). They formed differential
equations for different groups, drew graphs and studied the mortality curves to
find the model which was most likely to cause the outbreak of the plague
happened in different centuries.



Human fleas had the lowest BIC value in 7 out of 9 cities and they
were the quickest to spread the plague. Moreover, the difference between two
other models was more than 10, clearly out valuing and providing a strong
evidence against rat flea and pneumonic concepts. Therefore, human ectoparasite
model is the most preferred model to describe the pattern and the spread of
plague transmission.



The researchers had divided 9 cities for reliability, however, the
experimental data is too small to represent entire Europe. Also, they neglected
the common conditions that could have affected the death rate such as famine,
immunity, and war. Moreover, they did not mix the transmission routes,
preventing full description of plague contribution. The plague has occurred
many centuries ago so the accuracy of the research is impossible to be precise
(e.g. the number of rats). Furthermore, Monica H. Green, a historian at Arizona
State University claimed that this research has minimized the probability of
airborne transmission of disease by humans.  

On the other hand, Nils Stenseth, a professor at the University of
Oslo said that the spread of plague was too rapid if the rats were the cause.
Not only the number of dead rats was lacking which should have been significant
but also the rats should have been influenced by the plague first before the
humans, not the other way round if they were the main reason of the plague. 


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