The common tale for how the Black Death Pandemic spread through Europe is based on the rat and the flea. The rats arrived in European ports from Asia and their plague infested fleas spread through the medieval socities of Europe creating a pandemic that killed millions. The rats then established populations in Europe and their fleas stayed with them. Or did they? Research published in the Proceedings of the National Academy of Sciences journal challenges this hypothesis. Plague outbreaks in Asia seem to correlate with climate periods unfavourable for large rodent populations. This would have forced the fleas to seek alternative hosts, such as humans or camels. These hosts then transported the plague along the Silk Road from which it was transmitted to boats in Black Sea ports. Ten years after the outbreak in Asia, the plague would have arrived in Europe. This suggests multiple transmissions of the plague from Asia into Europe, rather than a European reservoir of plague. Multiple transmission events could also mean multiple strains.
The Black Death, originating in Asia, arrived in the Mediterranean harbors of Europe in 1347 CE, via the land and sea trade routes of the ancient Silk Road system. This epidemic marked the start of the second plague pandemic, which lasted in Europe until the early 19th century. This pandemic is generally understood as the consequence of a singular introduction of Yersinia pestis, after which the disease established itself in European rodents over four centuries. To locate these putative plague reservoirs, we studied the climate fluctuations that preceded regional plague epidemics, based on a dataset of 7,711 georeferenced historical plague outbreaks and 15 annually resolved tree-ring records from Europe and Asia. We provide evidence for repeated climate-driven reintroductions of the bacterium into European harbors from reservoirs in Asia, with a delay of 15 ± 1 y.