The World Health Organization says it still lacks clear answers about the death of a worker at a Russian institute that researched plague. The death, isolation of hundreds of coworkers and contacts, and reports of another employee with pneumonia have stirred concern about pneumonic plague.

One employee of the Irkutsk Research Anti-Plague Institute of Siberia in southeast Russia has died. Independent regional news outlet Lyudi Baikala identified the 28 year old worker as Darya Shipilova.

Shipilova went to a hospital in Shelekhov on September 29. The town is 12 miles southwest of Irkutsk, where she worked.

Rospotrebnadzor, Russia’s infectious disease agency, said she died from pneumonia of “unknown” origin. The agency played down suspicions that her death was connected to her work.

Media reports have described another institute employee as having pneumonia of “undetermined cause,” WHO Director-General Dr. Tedros Adhanom Ghebreyesus said Wednesday. WHO has also seen local media accounts describing a broken test tube inside the laboratory.

“I know what you know from the media of the test tube breaking and exposure,” said Dr. Maria Van Kerkhove, director of WHO’s department of Epidemic and Pandemic Management. WHO said it first learned about the patient through AI monitoring of local news reports about illnesses of concern.

Tedros said the Russian government held a meeting on October 2 “because they suspected a resident of the Irkutsk Institute had contracted a dangerous infection.” Under international law, Russia should have reported the incident within 24 hours.

After WHO requested more information, the Russian Federation responded Tuesday that “no case of plague had recently been reported.” Russian authorities also said about 200 people who had contact with the worker were quarantined or placed under close follow up.

WHO requested additional information but said it had not received it. “We don’t have the full picture of this event, and we are not able to conduct a full risk assessment,” Tedros said.

Hospitals in the area have restricted visitors and conducted comprehensive health checks. Some have required questionnaires about plague and symptoms of other infectious diseases, while Rospotrebnadzor has said the regional “epidemiological situation” remains stable.

About 95 people work at the Anti-Plague Institute, which opened in 1934 to research the disease. Yersinia pestis, the bacterium that causes plague, still circulates among wildlife in Siberia and neighboring regions.

More than 200 kinds of mammals, mostly rodents, can contract Y. pestis globally, according to studies. Marmots are the most common carriers in Siberia, and people are most often infected through exposure to infected rodents.

Based on limited information, WHO said the risk to the general public “appears to be low.” From 2019 through 2025, 10 countries reported almost 4,000 suspected human plague cases and 423 deaths.

Pneumonic plague is a rare and deadly lung infection, and it is the only form that can spread between people. Its basic reproduction number is about 1.1 to 1.3, meaning one infected person can spread the disease through coughing or sneezing to about 1.3 people.

Symptoms can appear within 24 hours of exposure, according to Dr. Rutul Dalal, an infectious disease physician at Penn State Health St. Joseph Medical Center. Patients treated during the first 24 hours of infection have the best chance of survival and typically receive at least 10 to 14 days of antibiotics.

Ciprofloxacin, levofloxacin, moxifloxacin, gentamicin and streptomycin typically work best against plague. Vaccines have been used since the 19th century, but their protection against pneumonic plague remains unclear, and WHO recommends vaccination only for high risk groups and laboratory workers regularly exposed to the bacteria.

The bacterium has infected human populations for at least 5,000 years and circulates on every continent except Oceania. Three of the deadliest pandemics were caused by Y. pestis, including the Black Death, which wiped out an estimated 30% to 60% of Europe’s population and killed an estimated 75 million to 200 million people globally.

The last documented person to person transmission of pneumonic plague in the United States occurred in Los Angeles in 1924. In July 2025, a person in Arizona became the most recent person in the United States to die from pneumonic plague and was probably infected while working with sick cats.