Findings for Northern China, a region with a dry climate, were completely different from those for Southern China, a region with a wet climate. The pattern in the arid north followed the conventional theory that wet conditions led to more cases of plague, but wetter conditions in the already humid south led to the opposite - fewer cases of plague.
The researchers surmise that as more rain is added to the already wet regions of Southern China, the resulting floods reduce rodent populations harboring fleas, cutting off the plague's path to human infection. But there is a lot still unknown.
Historically at least 17 strains of plague have existed on Earth since originating in rodents that lived in China more than 2,600 years ago. Last year, geneticists identified two strains of the bacterium Yersinia pestis as the cause of the "Black Death" pandemic that killed at least 25 million people in Europe from 1347 to 1665.
And the story isn't over. Three other strains still exist today.
"From 1954 to 1997," notes the PNAS study, "human plague was reported in 38 countries, making the case of plague a reemerging threat to human health." According to the Center for Disease Control, an average of 13 cases are reported annually in the western United States.