The bacterium that causes the plague, which killed millions of Europeans over four centuries from the 1350s, was repeatedly reintroduced from Asia and did not establish itself in European rodents as was thought.

Yersinia pestis bacteria live in wild rodents and can infect humans when climate changes cause rodent populations to collapse, triggering plague-carrying fleas to find alternative hosts. To locate plague reservoirs in Europe, Nils Christian Stenseth at the University of Oslo and his colleagues analysed historical outbreaks along with tree-ring-based records of climate. They found no connection between fluctuations in European climate and plague outbreaks, but did find links between Asian climate changes and outbreaks at European trade harbours.

The authors conclude that the plague took about 15 years to travel overland to Europe.

Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1412887112 (2015)