What is pneumonic plague and how does it spread?

The World Health Organization has asked Russia for more information on the death of a worker at a Siberian plague research laboratory.

It is still not clear exactly what led to the worker falling ill, but media reports have led to a flurry of speculation.

Russian officials are still investigating the cause of the death, but they say those who came into contact with the researcher all tested negative for pneumonic plague.

Pneumonic plague is a really serious lung infection, often described as the most virulent form of plague.

There are two other types of plague: bubonic, which affects the lymph nodes; and septicaemic, which affects the blood.

Pneumonic plague can occur after an initial infection of one of these other types.

Symptoms include shortness of breath, chest pain, and often coughing up blood.

It's regarded as being more serious but less common than bubonic plague.