Pneumonic plague is a rare but extremely serious lung infection caused by the bacterium Yersinia pestis.
It is considered the most severe form of plague and can cause symptoms including shortness of breath, chest pain and coughing up blood.
Plague still exists today, with an estimated 1,000 to 3,000 cases of all forms occurring globally each year.
Most cases are bubonic plague, but infections continue to occur in parts of Africa, Asia, the Americas and elsewhere.
Unlike other forms of plague, pneumonic plague can spread directly between people. An infected person can release respiratory droplets when coughing, which may infect someone nearby who breathes them in. This usually requires close contact.
People can also contract plague from infected fleas or animals, while other forms of the disease can sometimes spread to the lungs.
Pneumonic plague can progress rapidly, with an incubation period sometimes as short as 24 hours. Without prompt treatment, it is often fatal. However, antibiotics can effectively treat the infection when given early.
The disease has recently received renewed attention following the death of a worker at a plague research laboratory in Siberia. Russian authorities are investigating the case and say people who had contact with the researcher tested negative for pneumonic plague.
The World Health Organization has requested further information, while US and UK authorities have said there is currently no indication of a broader public health threat.
