Up to 3,000 cases of plague are reported each year

Researchers found that the bacterium that caused the deadly disease became more widespread following warmer springs and wetter summers.

The disease occurs naturally in many parts of the world, and the team hopes its findings will help officials limit the risk of future outbreaks.

The study appears in the Proceedings of the National Academy of Sciences.

The bacterium Yersinia pestis is believed to have triggered the Black Death that killed more than 20 million people in the Middle Ages.

Rodent hosts

The international team of scientists, who focused their research on Kazakhstan, said the disease was widespread among rodent populations.

Writing in the paper, co-author Nils Stenseth from the University of Oslo said: "The desert regions of Central Asia are known to contain natural foci of plague where the great gerbil (Rhombomys opimus) is the primary host.

"Plague spread requires both a high abundance of hosts and a sufficient number of active fleas as vectors transmitting plague bacteria between hosts," the Norwegian scientist added.

Fleas became active when the temperature exceeded 10C (50F), so a warm, frost-free spring led to an early start to breeding.

Plague is passed to humans through flea bites

The flea population continued to grow when the spring was followed by a wet, humid summer, the researcher wrote.

The combination of the two seasons' climatic conditions led to an increase in the number of the insects feeding off the great gerbils, resulting in a greater transmission of plague.

The study showed that just a 1C (1.8F) rise in the springtime temperature led to a 59% increase in the prevalence of the disease.

The greater prevalence of plague in the region's wildlife increased the risk of local people becoming infected.

Each year, up to 3,000 cases of humans contracting bubonic plague are reported in Asia, parts of Africa, the US and South America.

The researchers studied data on infected gerbils, flea counts and climate patterns from 1949 to 1995.

Professor Stenseth added that their findings also helped shed light on two of the world's worst plague outbreaks: the medieval Black Death and the Asian pandemic in the 19th Century, which claimed the lives of tens of millions of people.

"Analyses of tree-ring proxy climate data shows that conditions during the period of the Black Death (1280-1350) were both warmer and increasingly wet.

"The same was true during the origin of the Third Pandemic (1855-1870) when the climate was wetter and underwent an increasingly warm trend," he added.

The researchers hope their findings will help health officials put measures in place to limit the impact of future outbreaks.

But Professor Stenseth warned that recent changes to the region's climate suggested that warmer springs were becoming more frequent, increasing the risk of human infections.