It has long been suspected, and has recently been confirmed, that the Black Death of the fourteenth century was caused by the bacterium Yersinia pestis, which is carried by marmots, rats, and other rodents and spread predominantly by their fleas. The probable site of origin of the mid-fourteenth-century outbreak (also known as the "Second Plague Pandemic") was China. Historians have been greatly assisted by geneticists in confirming both the cause of the disease and a more precise location of its origin. It has most recently been suggested that the bacterium developed among the ground-dwelling rodents of the Tian Shan Mountains, which lie to the north of the traditional Silk Road.[1] And while that level of precision was formerly impossible, and even now is hypothetical, the notion that the Black Death spread along the Silk Road, carried by travelers and traders, is nothing new.[2]
Recent historical and scientific work has modified and perhaps overturned this hypothesis, at least as it pertains to the entry of the plague into Europe in the later 1340s. First, there is now strong evidence that the plague passed from rodents to humans, sparking an epidemic, much earlier than was formerly thought, in the thirteenth century rather than the fourteenth. While this did not make it to Europe, and thus has not been part of the standard (European-history driven) narrative, the outbreak does appear to have struck several Chinese cities far to the east in the thirteenth century.[3] The route of eastward transmission remains unknown, but something akin to the Silk Roads, broadly speaking, does seem likely.
On the other hand, transmission to the West now appears to owe less to the trickle-trade or few long-distance merchants of the Silk Road and more to the movements of Mongol armies.[4] Further -- and disappointingly to trebuchet enthusiasts everywhere -- one of the best-known stories about the Black Death, that it was spread by invading Tatars to the city of Caffa on the Black Sea when they hurled plague-infested bodies over the walls, has recently been shown to be a fabrication. Rather, the plague entered the city only after the seige was lifted.[5]
A 2020 map of the spread of the plague, incorporating the latest research and hypotheses, is shown here:
Caption: Draft Map Showing the Spread of Plague Through Eurasia and Africa Between the 13th and 15th Centuries.
Source: https://www.medievalacademy.org/page/webinars
[1] As of this writing, the best and most recent review of the state of knowledge is Monica Green, "The Four Black Deaths," American Historical Review 125.5 (December 2020), pp. 1601-1631.
[2] E.g. Nicholas Wade, "Europe’s Plagues Came From China, Study Finds", The New York Times, Oct. 31, 2010, available online at https://www.nytimes.com/2010/11/01/health/01plague.html. A more sophisticated and recent version of the Silk Road hypothesis is José M. Gómez and Miguel Verdú, "Network theory may explain the vulnerability of medieval human settlements to the Black Death pandemic", Scientific Reports 7 (March 2017), available online at https://www.nature.com/articles/srep43467.
[3] This was hypothesized by Robert Hymes in "Epilogue: A Hypothesis on the East Asian Beginnings of the Yersinia pestis Polytomy," The Medieval Globe: Vol. 1 : No. 1 , Article 12, available at https://scholarworks.wmich.edu/tmg/vol1/iss1/12, and further developed in a presentation in the webinar "The Mother of All Pandemics: The State of Black Death Research in the Era of COVID-19", hosted by the Medieval Academy of America on 15 May 2020. The whole webinar, over two hours in length, can be viewed here: https://youtu.be/VzqR1S8cbX8 . Hymes' paper begins around minute 23.
[4] See Green (note 1 above) for discussion.
[5] Hannah Barker, "Laying the Corpses to Rest: Grain, Embargoes, and Yersinia pestis in the Black Sea, 1346–48", Speculum 96/1 (January 2021), pp. 97-126.
-William Campbell