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By Michael Kahn

LONDON (Reuters) - Plague, the disease that devastated medieval Europe, is re-emerging worldwide and poses a growing but overlooked threat, researchers warned on Tuesday.

While it has only killed some 100 to 200 people annually over the past 20 years, plague has appeared in new countries in recent decades and is now shifting into Africa, Michael Begon, an ecologist at the University of Liverpool and colleagues said.

A bacterium known as Yersinia pestis causes bubonic plague, known in medieval times as the Black Death when it was spread by infected fleas, and the more dangerous pneumonic plague, spread from one person to another through coughing or sneezing.

"Although the number of human cases of plague is relatively low, it would be a mistake to overlook its threat to humanity, because of the disease's inherent communicability, rapid spread, rapid clinical course, and high mortality if left untreated," they wrote in the journal Public Library of Science journal PloS Medicine.

Rodents carry plague, which is virtually impossible to wipe out and moves through the animal world as a constant threat to humans, Begon said. Both forms can kill within days if not treated with antibiotics.

"You can't realistically get rid of all the rodents in the world," he said in a telephone interview. "Plague appears to be on the increase, and for the first time there have been major outbreaks in Africa."

Globally the World Health Organization reports about 1,000 to 3,000 plague cases each year, with most in the last five years occurring in Madagascar, Tanzania, Mozambique, Malawi, Uganda and the Democratic Republic of Congo. The United States sees about 10 to 20 cases each year.

More worrying are outbreaks seem on the rise after years of relative inactivity in the 20th century, Begon said. The most recent large pneumonic outbreak comprised hundreds of suspected cases in the Democratic Republic of Congo in 2006.

Bubonic plague, called the Black Death because of black bumps that sometimes develop on victims' bodies, causes severe vomiting and high fever. Victims of pneumonic plague have similar symptoms but not the black bumps.

Begon and his colleagues called for more research into better ways to prevent plague from striking areas where people lack access to life-saving drugs and to defend against the disease if used as a weapon.

"We should not overlook the fact that plague has been weaponized throughout history, from catapulting corpses over city walls, to dropping infected fleas from airplanes, to refined modern aerosol formulation," the researchers wrote. (Reporting by Michael Kahn; Editing by Maggie Fox and Ibon Villelabeitia)