Were We Wrong? Rats May Not Have Spread the Black Death

Fagjun | Published 2018-01-25 19:16

It’s common knowledge at this point that rats were instrumental in spreading the Black Plague in medieval Europe. However, it’s possible that this blame is actually misplaced.

If rats didn't spread the Black Plague, who—or what—did?

If rats didn't spread the Black Plague, who—or what—did?


The Black Death, a plague that spread throughout Europe between the years 1340 to 1400, killed over 20 million people before it ended. In the centuries since then, it was widely believed that black rats carried fleas that in turn carried bacteria called Yersinia pestis. These bacteria were blamed for causing the plague, which in turn led to people blaming rats for the spread of the deadly disease.


However, a new study questions the veracity of this explanation. Apparently, the idea that rats facilitated the transmission of the plague doesn’t fit in with historical records. Records from the time period do not show large rat populations dying off, and the Black Death was also much deadlier and spread at a much faster rate than modern outbreaks. This suggests that humans, not rats, may actually be to blame.

Mathematical Models

The Black Death swept through Europe and took 75 to 200 million lives. [Image via Alamy]

The Black Death swept through Europe and took 75 to 200 million lives. [Image via Alamy]


Researchers used mathematical equations to come up with three models of possible plague transmissions during a time period in which there were a series of disease outbreaks in Europe. One of these outbreaks was the Black Plague.


The first mathematical model assumed that rats were responsible for spreading the plague, by first spreading it to their fleas, which then spread it to people. The second model assumed that humans spread the disease through their own fleas and body lice. Lastly, the third model blamed airborne pathogens that spread the disease in a phenomenon called a pneumonic plague.


Information on plague deaths in nine different regions revealed that the second model, in which human parasites were the culprit, was true in seven of the nine regions. The first model involving the rats didn’t really match historical records, since the disease would have had to first go through the rats themselves. The pneumonic plague also did fit the data.


This means that lice and fleas from humans were the main culprits behind the spread of pandemics in the medieval times. There’s a 2011 study that supported this conclusion, while a 2015 study claimed that the weather was too cold and rainy for rats to be able to effectively spread diseases.

The Dirty Human

Improved hygiene ended the Black Plague's reign over Europe.

Improved hygiene ended the Black Plague's reign over Europe.


Still, this is unlikely to be the end of the discussion on the matter. It’s possible that a number of historians will disagree, since they hold that the disease-causing bacteria may have at least come from rats or at least another animal. Rutgers University history professor Nükhet Varlık in particular thinks that the human-to-human transmission theory is “plausible”. Varlık also criticized the research for being "exclusively on the European experience. Plague spread across Afro-Eurasia during the Black Death and continued on and off for several centuries." The researchers, however, think that their models could still be improved with the addition of more data.


The Black Plague was eventually beaten when people began to improve their hygiene. They began to regularly clean themselves and began to cover open sewers. Thus, the unhygienic human may have indeed been to blame instead of the flea-ridden rat.

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