Increasing the amount of land used for farming in Tanzania is also raising the risk of plague outbreaks, according to a study published online today in The American Journal of Tropical Medicine and Hygiene. Plague, caused by the bacteria Yersinia pestis, is passed to humans through rats and fleas. In areas that had recently been converted from forest to maize fields, researchers found almost twice as many rodents infested with plague-carrying fleas. And a rat species that carries the disease was 20 times more common in cropland than in the nearby protected forest the researchers sampled. Both population growth and a shift from herding to farming are driving a rapid expansion of farms in the region, the authors say.