By Dorsey Armstrong, Ph.D., Purdue University
There are good reasons to be doubtful of the claim that the Black Death was caused by the bacterium Yersinia Pestis. The scholars and scientists repeatedly return to its means of transmission. However, no one theory can adequately explain the timing, severity, and sudden disappearance of the plague.
Unanswered Questions
There are many reasons to doubt the theory of Yersinia Pestis as the underlying cause of the Black Death. For one thing, there seems to be no logical correlation between outbreaks and the seasons. For example, some of the worst outbreaks occurred during really hot Mediterranean summers in Italy, but that’s exactly when rat fleas were least likely to be thriving.
Also, the plague was reported to have occurred in Scandinavia in the dead of winter. But considering the harsh climate of this region, it’s unlikely that pathogenic fleas could have survived.
Scholars have tried to explain these discrepancies by making exhaustive arguments about the kinds of fleas that might have carried the plague, from black rat fleas to gray rat fleas, human fleas, brown rat fleas, etc.
This is a transcript from the video series The Black Death: The World’s Most Devastating Plague. Watch it now, Wondrium.
Rats May Be Innocent
A 2015 study from the University of Oslo suggests that black or other rats may not have been responsible at all. The study, published in the Proceedings of the National Academy of Sciences, was substantial and thorough, investigating the link between climate and plague outbreaks.
Utilizing the analysis of tree-ring data they demonstrated that plague outbreaks in Europe did not correspond to weather patterns in the continent. But the outbreaks seem to have a correlation to the weather in Asia, particularly corresponding to years when there were wet springs and warm summers.
Most scholars agree that the outbreak appears to have originated in Asia and then moved west along trade routes. But these weather patterns are not conducive to breeding black rats, so the dominant theory doesn’t really quite fit—unless we look for another rodent that carries fleas.
A New Contender
The authors of the study argue that the rodent was gerbil. They suggest that when weather conditions provided an ideal breeding climate for gerbils and their fleas, a predictable westward outbreak of plague followed along the Silk Road and other trade routes.
The significant contribution they made is their ability to correlate ideal gerbil breeding seasons in the east to the timing of outbreaks.
Based on this argument, the periodic waves of the disease were not due to a resurgence of the plague found in the rat populations that were already in existence in Europe. Instead, each outbreak was caused by a new wave of infection coming from the east after a particularly productive period by gerbils.
Learn more about how the plague traveled by sea across the Mediterranean.
The Case for Yersinia Pestis
There is no denying that Yersinia Pestis is still the most compelling theory explaining the Black Death. Many experts have pointed out that the plague of the late 19th century was a weak shadow of its 14th-century predecessor. The reason seems to be significant medical advances and a quicker and more coordinated global response to fight the outbreak.
In essence, the modern plague was just a quick blip on the radar, while the medieval one was a devastating event that nearly wiped out Europe’s population.
Some scientists have proposed that the modern plague was caused by an evolutionarily yet much weaker form of Yersinia Pestis. This would make sense because the medieval form of the plague was so virulent that unless it evolved into a less deadly form, it was at risk of wiping itself out by killing all its potential hosts.
The Findings Based on the DNA
In a series of articles published since 2000, several different scientists have written reports about what they’ve discovered using DNA analysis of corpses excavated from plague cemeteries, focusing particularly on what they’ve been able to discover by analyzing remains (such as tooth pulp).
Several of these studies have investigated skeletons from throughout Europe and not just from one particular area, to make certain they weren’t generalizing from one aberrational mass grave.
Learn more about how the Black Death transformed the world.
Were There Two Distinct Waves of the Plague?
A study published by the journal PLOS Pathogens in 2010 found the DNA and protein signatures in a variety of skeletons from throughout northern, central, and southern Europe and also identified two previously unknown genetic branches of Yersinia Pestis associated with specific mass graves.
This suggests that the plague came to Europe in two distinct waves, and interestingly, these two variants now appear to be extinct. In other words, they burned themselves out.
Although this theory goes some way to explaining some of the puzzles surrounding the plague, like all other theories, it leaves many questions unanswered. But science continues to make great strides in this area, and we can hope that in the future, more answers will be forthcoming.
Common Questions about Bubonic Plague as the Underlying Cause of the Black Death
A study published by the journal PLOS Pathogens in 2010 found the DNA and protein signatures in a variety of skeletons from throughout northern, central, and southern Europe and also identified two previously unknown genetic branches of Yersinia Pestis associated with specific mass graves.
Many experts have pointed out that the plague of the late 19th century was a weak shadow of its 14th-century predecessor.
There are many reasons to doubt the theory of Yersinia Pestis as the underlying cause of the Black Death. For one thing, there seems to be no logical correlation between outbreaks and the seasons.
