During the British Empire, people used to be employed particularly to count the number on fleas on rats

Dr Ian Burgess

The Black Death, which cut the population of Europe by a third during the Middle Ages, is still a health problem in some parts of Asia and Africa. The infection, Yersinia pestis, is carried by fleas which usually live close to, and feed off rats.

Researchers at Cambridge University used a computer to model the movements of rats and fleas in a population of 60,000 rodents.

Old advice

The scientists found that if a rat cull is carried out after cases of plague have emerged in humans, the resulting scarcity of rodents means the fleas are forced to look for another host, often ending up with a human to bite.

Researchers say killing rats is not the answer to beating disease

This would cause the disease to spread like wildfire.

The research team warn: "If the cull is brought into effect after the first human cases have been reported, then this action can create a far larger force of infection for humans."

An insect specialist from the UK said that the Cambridge work confirmed the advice of rat experts as much as a century ago.

Normal habitats

Dr Ian Burgess, from the Medical Entomology Centre, said: "Rat fleas tend to stay in rat nests, rather than ride around on the rat.

"However, when the overall population of rats falls - perhaps due to plague - the number of fleas on each rat increases.

"During the British Empire, people used to be employed particularly to count the number on fleas on rats. The more fleas, the more chance of an outbreak."

The threat from rats and their fleas increases normally when rats are driven from their normal habitats outside towns by harvesting, natural disasters or lack of food, he said.