A new study of one of the oldest scourges in history - human plague - shows how variations of weather can set in motion environmental changes that can lead to disastrous effects.
An international team led by Chinese scientists took a hard look at a plague outbreak known as the Third Pandemic, which killed an estimated 2.2 million people in China from the 18th Century to the early 20th Century - comparing plague data with measures of yearly wetness and dryness going back 500 years.
Researchers in the field have a line of thought that goes like this: The plague bacterium, Yersinia pestis, is carried by rodents, so wetter conditions lead to greater vegetation growth, which produces more food for rodents. More rodents means more fleas, which bite the infected rodents and become carriers themselves, passing the disease on to humans.
But the analysis, just published in the Proceedings of the National Academy of Sciences, shows a picture that is more complex. Underlying climate conditions trump precipitation patterns.
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.
IMAGE: Roman doctor's dressed in elaborate robes and beaked masks to protect themselves from infections during the "Black Death" plague epidemic of the Middle Ages. Copper engraving by Paul Fuerst in 1656, via Wikipedia.
MAP: Courtesy U.S. Center For Disease Control.