The team reasons that large-scale climate fluctuations in Asia caused rodent colonies to collapse. Desperate fleas sought new hosts, jumping aboard passing animals, and plague got a free ride across the continent. It was a cycle that often repeated itself. Schmid’s team pinpointed three major pulses of plague introductions in Europe, coinciding with the continent’s three centuries of Black Death.
Climate shifts created the right conditions for the westward advance of plague, but it was humans who likely gave it the highways to travel. At that time, the extensive trade network known as the Silk Road ran throughout Eurasia. The trade routes were dotted with caravanserai, mass encampments where travelers could eat, lodge, buy supplies and refresh their camels, a favorite host of the fleas that carry Y. pestis.
“Camels appear to get less sick from plague than humans, and thus might last longer before they succumb,” Schmid says. An infected camel could carry fleas far along the route before dying. The caravanserai were likely critical in plague dispersal.
While the disease remains a modest threat in rural, undeveloped areas, Green warns that our current stable relationship with plague in the developed world is only as good as our control of urban rodent populations. Even then, outside of our cities, the occasional unfortunate brush with Y. pestis will occur, as the fate of the Park Service biologist reminds us.
Human populations are more mobile than ever, and our travel patterns combined with unchecked climate change only exacerbate the risk of outbreaks. If global warming disrupts the status quo, we may again see Y. pestis take deadly advantage of highly trafficked areas to wreak havoc on our population. “We will never be able to fully let our guard down,” Green says.
[This article originally appeared in print as "A Profile of Plague."]