What Is Pneumonic Plague? Understanding the Deadliest Form of a Disease That Never Disappeared

The World Health Organization has formally requested additional details from Russian authorities following the death of an employee at a Siberian laboratory dedicated to plague research. While the precise sequence of events that caused the worker to fall ill remains unconfirmed, the incident has generated considerable speculation in media reports and renewed global attention on one of humanity's oldest and most feared infectious diseases.

Russian officials continue to investigate what caused the researcher's death, but they have stated that all individuals who had contact with the employee tested negative for pneumonic plague. The Russian authorities also issued a statement noting that tests had found "no microorganisms associated with the patient's professional activities," leaving the exact cause of death still unclear at this stage.

So what exactly is pneumonic plague, and how concerned should the public be?

Pneumonic plague is a severe infection of the lungs and is frequently characterised as the most virulent form of plague disease. It stands apart from two other recognised types: bubonic plague, which primarily affects the lymph nodes, and septicaemic plague, which attacks the blood. Pneumonic plague can also emerge as a secondary complication following an initial infection of either of those other forms.

The symptoms of pneumonic plague are serious and can escalate rapidly. Patients typically experience shortness of breath and chest pain, and they often cough up blood. Compared with bubonic plague, pneumonic plague is regarded as more dangerous but also less common.

Globally, the number of plague infections of all three types fluctuates from year to year, but the total is estimated to fall somewhere between 1,000 and 3,000 cases annually. The vast majority of these are bubonic plague, and many occur in central Africa. Cases also arise in other parts of the world, including Russia, India, China, and the United States.

Professor Piero Olliaro, Professor of Poverty-Related Infectious Diseases at the University of Oxford, emphasised that the disease has not been consigned to history. He explained that the bacterium responsible, Yersinia pestis, continues to persist in animal reservoirs across many regions of the world. Around 200 rodent species can harbour the bacterium, with reservoirs present throughout the Americas, Eurasia, and Africa. As a result, sporadic human cases continue to appear, though established surveillance systems, infection-control measures, and effective antibiotic treatments remain essential tools for limiting their impact.

Plague is an infectious disease caused by bacteria typically found in small mammals and their fleas. Humans can contract it through the bite of an infected flea or by handling an infected animal. However, pneumonic plague can also spread from person to person through respiratory droplets. When an infected individual coughs, droplets carrying the bacteria are released into the air and can be inhaled by another person, although this mode of transmission generally requires quite close contact.

There is another pathway to developing pneumonic plague: a person who has had septicaemic plague can see the infection spread into the lungs, resulting in the pneumonic form of the disease. Historically, it is believed that pneumonic plague was the predominant type behind the Black Death, the catastrophic outbreak that killed millions of people across Europe during the 14th Century.

Dr Zania Stamataki, Associate Professor of Viral Immunology at the University of Birmingham, stressed the importance of allowing researchers to study diseases such as plague. She acknowledged that the idea of plague being present in a laboratory can seem alarming to the public, but she explained why such work is necessary. Plague is not merely a historical curiosity, she noted; 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 depend on laboratories that hold and study the live organism. This work, she said, is vital for preparing against future outbreaks.

According to the World Health Organization, pneumonic plague has an exceptionally short incubation period, sometimes as brief as 24 hours. Experts caution that if the disease is not diagnosed and treated promptly, it is nearly always fatal. The encouraging news is that plague responds well to antibiotics when treatment is initiated early.

Dr Malick Gibani, Clinical Associate Professor in Bacterial Vaccinology at Imperial College London, described the infection as a widely recognised serious threat to public health. He noted that pneumonic plague is a very severe manifestation of plague disease. Some textbooks state that it carries a 100% mortality rate if left untreated, although obtaining accurate modern figures is difficult. It is treatable with antibiotics, particularly when treatment begins early. For this reason, individuals with high-risk potential exposure may be offered post-exposure prophylaxis with antibiotics and/or quarantine.

In the Russian case, it remains unclear whether the person who died was actually infected with pneumonic plague. The Russian authorities have said that tests showed no microorganisms associated with the patient's professional activities, so the full picture of what happened has yet to emerge.

The incident has drawn attention at the highest levels of government. US President Donald Trump said Washington was looking "very strongly" at the case, while the Centers for Disease Control stated there was "no indication of a broader threat to the United States." The UK's Health and Safety Authority confirmed it was seeking more information but assessed the risk to the British population as "very low."

The episode serves as a reminder that plague, though rare and treatable, has not vanished. Its continued presence in animal reservoirs around the world means that sporadic human cases will keep occurring, making surveillance, rapid diagnosis, and early antibiotic treatment as important as ever.