The bacteria that cause plague, or the Black Death, could be lying dormant in common soil and water sources, posing a serious public health risk, scientists have warned.

The finding, published in the journal Emerging Infectious Diseases, could explain why plague suddenly re-emerges without warning in countries such as Madagascar and even the United States.

In the middle ages the Black Death swept through Europe, killing an estimated 75 to 200 million people. It is no longer a global threat but the new research may explain its occasional outbreaks across the world.

David Markman, from Colorado State University who led the study, said that plague is endemic in many different parts of the world and its origins are still not well understood.