Flooding in central China has created an unexpected nightmare. When the Yangtze flooded on June 23, the flood-waters flushed out millions of rats, which have spread out and ravaged farmland.

Centuries ago a similar outbreak led to a far greater catastrophe. In 1332 the Yellow River burst its banks and drowned millions of people, and the following year severe drought triggered famine. These disasters set the stage for rats to swarm out carrying the virulent plague disease in their fleas. The rats spread out of China and the plague eventually consumed Asia and Europe in the Black Death, possibly the worst pandemic in history. A third of the population of Europe died in two years, which led to drastic upheavals in medieval society.

Since then the plague has erupted repeatedly after dramatic lurches in climate, including the Great Plague of London of 1665, when about one-fifth of the capital’s population died.

In Kazakhstan and Mongolia, about ten human cases of plague are reported each year. Natural populations of gerbils in these countries act as a reservoir for the disease.

The fear is that warmer spring temperatures and wetter summers brought on by climate change in central Asia, are helping to boost the population of gerbils and also spread Yersinia pestis, the bacterium that causes bubonic plague.