He says the rat story doesn’t add up: If rats carried plague to Europe, and Europe is still full of rats today, then plague should also be found in European cities. But it isn’t.

Stenseth suspects that the plague came to Europe multiple times from Asia, where it still exists today. The rodents that carry plague in Asia include the cutest of infectious hosts: the gerbil.

“What we are suggesting is that it was gerbils in Central Asia and the bacterium in gerbils that eventually came to Europe,” Stenseth says. The scientists used climate records to check their theory, and they found a tentative link. When the climate in Asia was good, gerbils are thought to have thrived; but when it went bad, the population crashed. And about 15 years after each boom and bust, a plague outbreak erupted in Europe. The theory is that fleas carrying plague jumped from dead gerbils to pack animals and human traders, who then brought it to European cities. The research team’s results appear in the current issue of the Proceedings of the National Academy of Sciences.