What is pneumonic plague and how does it spread?

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.

The global number of infections of all three types of plague varies each year, but it is probably somewhere between 1,000 to 3,000 cases.

Most of these are bubonic plague, and many of them occur in central Africa, but cases also develop in other places, including Russia, India, China, and the United States.

Prof Piero Olliaro is Professor of Poverty-Related Infectious Diseases at the University of Oxford.

He said: "Plague itself has not disappeared: the bacterium that causes it, Yersinia pestis, persists in animal reservoirs across many parts of the world.

"Around 200 rodent species can harbour the bacterium, with reservoirs found across the Americas, Eurasia and Africa.

"Sporadic human cases therefore continue to occur, but established surveillance, infection-control measures and effective antibiotic treatment are important tools for limiting their impact."

Plague is an infectious disease caused by bacteria usually found in small mammals and their fleas.

You can catch it by getting bitten by an infected flea or handling an infected animal.

But human-to-human transmission of pneumonic plague is also possible through coughing infected droplets into the air, which are then breathed in by another person, although this normally requires quite close contact.

You can also develop it if you have had septicaemic plague and the infection then spreads into the lungs.

It's thought that pneumonic plague was the main type of plague behind what is known as the Black Death, the massive outbreak that killed millions of people across Europe in the 14th Century.

Dr Zania Stamataki, Associate Professor of Viral Immunology at the University of Birmingham, says it is important that researchers are able to study diseases like plague.

"I understand why people find the idea of plague in a laboratory alarming, so it is worth saying why that work exists," she said.

"Plague is not a historical disease. It remains endemic in rodent populations across Africa, Asia and the Americas, and human cases occur every year, including outbreaks of the pneumonic form.

"Diagnostics, vaccine development and surveillance for antibiotic resistance all require laboratories that hold and study the live organism.

"This work is vital to prepare against outbreaks."

According to the World Health Organization, pneumonic plague has a very short incubation period - sometimes just 24 hours.

If it isn't diagnosed and then treated quickly, experts warn it is nearly always fatal.

But the good news is that plague responds well to antibiotics if the treatment starts early.

Dr Malick Gibani, Clinical Associate Professor in Bacterial Vaccinology at Imperial College London, says the infection is widely recognised as a serious threat to public health.

"Pneumonic plague is a very severe manifestation of plague disease.

"Some textbooks will say it has 100% mortality if left untreated, and it's difficult to have accurate figures in the modern age.

"It is treatable with antibiotics, particularly if started early.

"That's why anyone with high-risk potential exposure might be offered post-exposure prophylaxis with antibiotics and/or quarantine."

In the Russian case, it is still not yet clear whether the person who died was infected by pneumonic plague.

The authorities in Russia have issued a statement saying that tests had shown "no microorganisms associated with the patient's professional activities" – and so at this stage it is still not quite clear what has happened.

US President Donald Trump said Washington was looking "very strongly" at the case, while Secretary of State Marco Rubio said there was no cause for alarm.

The UK's Health and Safety Authority said it was seeking more information, but that the risk to Britain's population was "very low".