Rats have been blamed for spreading the plague for more than a century, but new research suggests the real culprit was their cousin the gerbil.
Black rats were thought to have brought the plague bug (Yersinia pestis) along the Silk Road to Europe from the Far East, causing a series of outbreaks from 1347 on.
Fleas (Xenopsylla cheopis) caught it from their hosts, the theory went, and they in turn transmitted it to humans.
But if that were the case, you would expect plague to break out whenever the weather was favourable to rats.
And that wasn't the case, according to a team led by Professor Nils Christian Stenseth from the University of Oslo.
"You would need warm summers, with not too much precipitation. Dry, but not too dry," Professor Stenseth told the BBC. "We have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather."
At least in Europe.
But there is a correlation with a wet spring followed by a warm summer in Asia.
"Such conditions are good for gerbils. It means a high gerbil population across huge areas and that is good for the plague," Professor Stenseth said. Not surprisingly, the fleas also do well in that sort of weather.
"We show that wherever there were good conditions for gerbils and fleas in central Asia, some [14 to 16] years later the bacteria shows up in harbour cities in Europe and then spreads across the continent."
This fits with research reported last year which showed that different strains of plague had entered Europe several times.
Previously it was thought that the bacterium arrived once, and stayed for four centuries, surviving in a rat reservoir between human epidemics.
As the authors of the latest article put it in Proceedings of the National Academy of Sciences "This association strongly suggests that the bacterium was [continually] re-imported into Europe during the second plague pandemic, and offers an alternative explanation to putative European rodent reservoirs for how the disease could have persisted in Europe for so long."
The researchers compared a dataset of 7,711 historical plague outbreaks with tree-ring records from Europe and Asia.
As well as Professor Stenseth, the paper was co-written by Boris Schmid, Ryan Easterday, and Barbara Bramanti of the Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo; Ulf Buntgen and Christian Ginzler of the Swiss Federal Research Institute for Forest, Snow and Landscape; and Lars Walloe of the Department of Physiology, Institute of Basic Medical Sciences, University of Oslo.
