The 28 year old worker, identified as Darya Shipilova, worked at the Irkutsk Research Anti Plague Institute in southeastern Russia. She was admitted to hospital on September 29 in Shelekhov, around 12 miles southwest of Irkutsk, and died on October 2.

Russian health authorities said she died from pneumonia of unknown origin and played down suggestions that her illness was linked to her work.

The WHO said media reports also indicated that a second employee may have developed pneumonia of undetermined cause. The organisation said it had requested further information from Russian authorities but had not received a complete response.

WHO Director-General Tedros Adhanom Ghebreyesus said Russia reported that no recent case of plague had been identified and that around 200 people who had contact with the deceased worker had been placed under quarantine or close medical observation.

The WHO said it first became aware of the incident through its monitoring of local news reports. It also said it had seen reports about a broken test tube and possible exposure at the institute.

The Anti-Plague Institute employs around 95 people and was established in 1934 to conduct research on plague. The bacterium that causes the disease, Yersinia pestis, continues to circulate among wildlife in Siberia and neighbouring regions.

The WHO currently considers the risk to the general public to be low based on the limited information available.

Pneumonic plague is a rare and potentially fatal form of plague that affects the lungs. It is the only major form of plague that can spread directly from person to person, usually through respiratory droplets.

The WHO said patients treated within the first 24 hours have the best chance of survival. Plague is generally treated with antibiotics for 10 to 14 days, with drugs including ciprofloxacin, levofloxacin, moxifloxacin, gentamicin and streptomycin among those used to treat the infection.

WHO data show that 10 countries reported almost 4,000 suspected human plague cases and 423 deaths between 2019 and 2025.

Plague has affected human populations for thousands of years and caused three major pandemics, including the Black Death, which killed millions of people in Europe, the Middle East and other regions.

Despite its history, plague remains treatable with antibiotics. The WHO says vaccination is generally not recommended except for people at high risk, including laboratory workers who are regularly exposed to the bacterium.

For now, the WHO says it cannot complete a full risk assessment because of the lack of detailed information about the incident in Russia.