A major wave of some extremely virulent disease killed between roughly one-third and two-thirds of Europeans within a few years in the mid-fourteenth century. Bubonic and pneumonic plague are widely thought to be the cause of this and subsequent waves of the "Black Death."
However, there are questions that remain unanswered. Two of them are that the Black Death has occurred in the absence of rats (which are now known to be the host of plague-carrying fleas), and that the symptoms attributed to historical episodes of the Black Death have not always corresponded with the known symptoms of modern bubonic and pneumonic plague.
These and other inconsistencies are reason to question a long-held conclusion. Might the Black Death have been caused by something other than bubonic and pneumonic plague?
Mark Welford and Brian Bossak (Georgia Southern University) have added fuel to this debate. They have found that the seasonal peaks in European deaths during Black Death epidemics don't correspond with the seasonal peaks in deaths from modern outbreaks of bubonic and pneumonic plague.
Historical Black Death and modern bubonic and pneumonic plague.
It can be very challenging to attribute a past disease to a currently-identified pathogen. This is because historical records aren't necessarily accurate, and scientific knowledge is far more advanced than it was hundreds of years ago.
However, plenty of accurate records do exist. The scientists studied translated daily, weekly, and monthly death records in towns located in France, England, Spain, Hungary, and Russia, compiled during historical episodes of the Black Death.
They also studied mortality records from recent (late 1800s and early 1900s), known episodes of bubonic and pneumonic plague, in India and Mongolia. The scientists compared these two large datasets, specifically to compare monthly peaks in mortality.
The scientists found that across all of the historical records they studied, mortality associated with the Black Death peaked in mid-July. In contrast, mortality associated with known cases of modern bubonic and pneunomic plague have peaked in February.
Implications.
The scientists offer several explanations for this discrepancy. One possibility is that the bacteria which cause bubonic and pneumonic plague today are genetically different from historical variants of the bacteria.
Another possibility is that people were simply exposed to bubonic and pneumonic plague at different times of year back then, compared to modern times. They discount a third possibility of climatic changes being responsible for the discrepancy, because their historical records cover a space of hundreds of years in a wide range of locations.
Their fourth proposed possibility is that the Black Death may not have been caused by bubonic and pneumonic plague. The scientists specifically state that there is no hard evidence to conclusively support or refute this fourth possibility.
They suggest that interdisciplinary research involving anthropologists, historians, doctors, and other professionals will be required to test all of the possibilities. It's too bad that a modern immunology lab can't be transported (Star Trek-style) back to the European Middle Ages; that would clear up much confusion.
In the meantime, it's clear that the Black Death can't be conclusively attributed to bubonic and pneumonic plague. This research also has modern implications, specifically towards understanding how a future pandemic may spread across the modern world.
for more information:
Welford, M. R., & Bossak, B. H. (2009). Validation of Inverse Seasonal Peak Mortality in Medieval Plagues, Including the Black Death, in Comparison to Modern Yersinia pestis-Variant Diseases PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008401