The death of a researcher at a plague laboratory in Siberia has prompted the World Health Organization to press Moscow for answers.

Russian officials have yet to share details of what caused the worker’s illness, and this information vacuum has fuelled speculation .

But what is the disease, and how much of a threat does it pose ?

What is pneumonic plague?

Plague is a rare and serious infection caused by the bacterium Yersinia pestis , usually found in small mammals such as rodents and their fleas. Pneumonic plague is the most virulent of plague’s three forms, and is a severe infection of the lungs.

The other types are bubonic plague – the Black Death that laid waste to much of Europe during the Middle Ages – which attacks the lymph nodes, and septicaemic plague, which attacks the blood.…