It has conventionally been accepted for years that rats were the primary reason for the spread of bubonic plague, particularly the outbreak of the "Black Death" in 1347. But Nils Christian Stenseth at the University of Oslo, Norway has been working with a team to clear the rats of responsibility. Experts believe that he may have done it. 

Stenseth has aligned the data of historical temperature changes and the outbreaks of plague in Europe and Asia. What he found is that it's far more likely that Central Asian rodents like gerbils or marmots were most responsible for the plague's spread because outbreaks would align with dramatic temperature swings in the region. When temperatures rise, so do the number of gerbils for plague-carrying fleas to inhabit. When the temperatures drop, the rodents die and the fleas have to find new hosts, like camels or humans.

Hartmut Dunkelberg of the University of Goettingen in Germany tells New Scientist that he finds the study convincing. "Climate influences different factors such as the development of fleas and the distribution of plague reservoirs," he says. "Many human infections are seasonal."