What is pneumonic plague? A guide to the disease back in the headlines

Plague is one of history's most notorious diseases, blamed for pandemics that killed tens of millions of people over the centuries. It has returned to the headlines this month following the reported death of a laboratory worker in Russia, renewing public attention on a disease many associate purely with the past.

What causes it

Plague is caused by the bacterium Yersinia pestis, which is typically carried by rodents and spread to humans through the bite of infected fleas. The disease takes several forms depending on how it enters the body. Bubonic plague, the most common form, causes painful swelling of the lymph nodes. Septicemic plague occurs when the bacteria enter the bloodstream. Pneumonic plague, the form now drawing attention, develops when the bacteria infect the lungs, either after bubonic or septicemic plague spreads untreated or through direct inhalation of infectious droplets.

Why pneumonic plague is more dangerous

Unlike the other forms, pneumonic plague can spread directly between people through respiratory droplets, similar to other lung infections, making it the only form of plague capable of person-to-person transmission through the air. It is also the fastest-moving and most lethal form if untreated, with symptoms including fever, cough, chest pain and difficulty breathing that can progress rapidly. Health authorities say that without prompt antibiotic treatment, pneumonic plague is close to universally fatal, though modern antibiotics are highly effective when administered early.

How common is it today

Plague has not disappeared. Health agencies record a small number of cases most years, concentrated in parts of Africa, Asia and the Americas where the bacteria remain present in wild rodent populations. Laboratory-acquired infections, like the case reported in Russia, are rare but have occurred previously in facilities that study the bacterium for research purposes. Public health officials emphasize that modern surveillance, rapid diagnosis and antibiotic treatment have made plague a largely manageable disease in places with functioning health systems, even though it remains fatal in the rare instances when treatment is delayed.