The Method The researchers digitalized publicly available data

  The outbreak of The Great Plague (BlackDeath) caused by humansIntroductionThe Black Deathwhich occurred in the 14th Century has killed one-third of the totalpopulation in Europe, and the flea (Yersinia pestis, bacterium) living in rats were blamed forthe spread of the disease for 700 years.

However, a research that carried outin 2017 claimed that human ectoparasites were more likely to have caused therapid spread of the disease than the rat flea, ultimately challenging thecommonly known concept of spreading of the plague.  SourceTheUniversity of Oslo and University of Ferrara’s researchers: KatharineR. Dean, FabienneKrauer, Lars Walløe, OleChristian Lingjærde, BarbaraBramanti, NilsChr.

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Stenseth, Boris V. Schmid reported through Proceedingsof National Academy of Sciences. BBC reported this news on the 16thJanuary 2018.  MethodThe researchers digitalized publicly available data from 9 citiesthat were affected by plague outbreaks.

Then, theydeveloped a Susceptible–infectious–recovered(SIR) model for plague transmission. Each of the cities wasdivided into 3 groups- Human-Ectoparasite Model (human fleas), Pneumonic PlagueModel (direct human-to-human transmission) and Rat-Flea Model (rat fleas). Theydeveloped differential equations for each group, drew a graph according to theresults and studied the mortality curves to find the most-fitting model foreach outbreak of the plague happened in different centuries.

 FindingsHuman fleas had the lowest BIC value in 7 out of 9 cities and theywere found to spread the plague fastest. Moreover, the difference between humanmodel and two other models was bigger than 10, clearly out valuing and providinga powerful evidence against rat flea and pneumonic concepts. Therefore, humanectoparasite model is the most preferred model to describe the pattern and thespread of plague transmission.  ReliabilityThe researchers had divided 9 cities for reliability, however, theexperimental data is a bit too small to represent entire Europe. Also, they neglectedthe local conditions that could have affected the death rate such as famine,war, and immunity. Moreover, they did not mix the transmission routes,preventing full description of plague contribution.

The plague has occurredmany 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 ArizonaState University claimed that this research has minimized the probability ofairborne transmission of disease by humans.

 On the other hand, Nils Stenseth, a professor at the University ofOslo 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 significantbut also the rats should have been influenced by the plague first before thehumans, not the other way round if they were the main reason of the plague.