By Jen Christensen, CNN
(CNN) — The World Health Organization said Wednesday that it had few clear answers about the recent death of a worker at a Russian institute that researched plague. The death, the subsequent isolation of hundreds of co-workers and close contacts, and reports that another worker may have pneumonia have fueled global concern about a rare form of plague.
Here’s what we know about pneumonic plague by the numbers.
The situation in Russia
One person who worked at the Irkutsk Research Anti-Plague Institute of Siberia in southeast Russia has died.
There have also been media reports that a second employee of the lab has pneumonia of “undetermined cause,” the director-general of the World Health Organization said Wednesday.
The 28-year-old who died was identified by the independent regional news outlet Lyudi Baikala as Darya Shipilova.
Shipilova first went to the hospital September 29 in Shelekhov, a town 12 miles southwest of Irkutsk, where she worked.
Three days later, on October 2, she died.
Rospotrebnadzor, the Russian agency that handles infectious disease, says the worker died of pneumonia of “unknown” origin and downplayed suspicions that her death was related to her work.
WHO said it has seen about local media reports about an incident in the lab involving a broken test tube. Currently, “I know what you know from the media of the test tube breaking and exposure,” Dr. Maria Van Kerkhove, director of WHO’s department of Epidemic and Pandemic Management, said Wednesday.
On October 2, the Russian government held a meeting “because they suspected a resident of the Irkutsk Institute had contracted a dangerous infection,” WHO Director-General Dr. Tedros Adhanom Ghebreyesus said Wednesday.
WHO said it first heard about the patient because it uses AI to monitor local news reports for illnesses of concern.
Under international law, Russia should have reported the incident within 24 hours.
After the WHO sought more information about the death, the Russian Federation replied Tuesday to say that that “no case of plague had recently been reported.”
The Russian government also said it placed about 200 people who’d had contact with the worker in quarantine or under close follow-up, WHO said.
WHO said it had requested further information from the government, but has 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. “Timely, complete and transparent information sharing under the international regulations is essential to clarify conflicting reports and enable an accurate assessment of potential public health risks.”
A number of hospitals in the area have restricted visitors and are conducting comprehensive health checks, including requiring people to fill out questionnaires about “plague” and other infectious disease symptoms. Rospotrebnadzor, however, reassured people in the region where the lab operates that the “epidemiological situation” is stable.
Plague research
About 95 people work at the Anti-Plague Institute.
The institute opened in 1934 to research plague. The bacterium that causes plague, Yersinia pestis, still circulates among wildlife in Siberia and neighboring regions.
Globally, more than 200 kinds of mammals, mostly rodents, can contract Y. pestis, studies show. Marmots are the most common carriers in Siberia.
WHO said the risk to the general public “appears to be low” at this time based on the limited information it has.
What to know about plague
There are thousands of suspected plague cases around the world every year, Van Kerkhove said.
From 2019 to 2025, 10 countries reported almost 4,000 suspected human plague cases and 423 deaths, WHO said Wednesday.
Central Asia has the largest number of foci, or areas where the disease is present. People are most often infected after exposure to infected rodents.
There are four major types of plague caused by Y. pestis that attack different parts of the body:
Pneumonic plague is a rare, deadly lung infection. Scientists consider pneumonic plague the most dangerous form of the infection. Bubonic plague, the most common form, often develops after a flea bite or when the bacteria enters through a cut or break in the skin. The infection triggers swollen, painful lymph nodes and can progress to septicemic plague. Septicemic plague infects the bloodstream, potentially leading to organ failure. It can also come directly from a flea bite. Pharyngeal plague attacks the throat after someone eats infected meat.WHO said it conducted an initial assessment of risk based on an understanding that the Russian lab worker’s case could be pneumonic plague.
Pneumonic plague is the only form of the bacterial infection that can spread from person to person.
The basic reproduction number is about 1.1 to 1.3, meaning that one person with pneumonic plague can spread the disease through a cough or sneeze to about 1.3 people, studies show. Droplets from infected animals can also transmit it.
People can develop symptoms as soon as 24 hours after exposure, according to Dr. Rutul Dalal, an infectious disease physician at Penn State Health St. Joseph Medical Center. Typically, people with pneumonic plague are so sick that they rarely spread it because “it’s very difficult for someone to even walk, do their daily activities, let alone travel,” he said.
There are no direct flights between Russia and the US, Dalal noted. By the time infected travelers reached the US, they would already be showing symptoms.
“Of course, you have to be careful,” he said, because “the world is a little village now,” as everyone learned during the Covid-19 pandemic.
Patients treated within the first 24 hours of infection have the best chance of surviving.
People with plague typically get at least 10 to 14 days of antibiotics.
Five antibiotics typically work best against plague: ciprofloxacin, levofloxacin, moxifloxacin, gentamicin and streptomycin.
Doctors have used plague vaccines since the 19th century, but it’s not clear how protective they are against pneumonic plague. WHO doesn’t recommend vaccination except for high-risk groups and lab workers who are regularly exposed to the bacteria.
History of plague
The bacterium that causes plague circulates on every continent except Oceania.
It has infected human populations for at least 5,000 years, studies of archaeological sites show.
Three of the world’s deadliest pandemics were caused by Y. pestis.
The first known plague pandemic, from 541 to 750, is known as the Plague of Justinian because it weakened Emperor Justinian’s attempts to reunite the Roman Empire, and some historians believe it contributed to the empire’s downfall. The pandemic probably began with fleas that bit rodents in Africa, and the disease raced along trade routes, killing up to 100 million people. Some historians believe that it killed half the population of the Mediterranean, although some contemporary historians say it may have been smaller than earlier historians believed. The second plague pandemic, from 1347 through the 1700s and described by historians as the Black Death, probably originated in Central Asia and swept through Europe and the Middle East along trade routes. It wiped out about 30% to 60% of Europe’s population, an estimated 25 million to 50 million people, and is believed to have killed 75 million to 200 million globally. The third plague pandemic, from 1894 to 1901, began in China and traveled along trade routes by steamship, striking India and China hardest and killing 12 million to 15 million people, mostly in Asia.1924 was the year of the last documented person-to-person transmission of pneumonic plague in the US, during an outbreak in Los Angeles.
Since the 1990s, most human cases have been in Africa, according to WHO.
July 2025 was when the last person in the US died from pneumonic plague. The person lived in Arizona and was probably infected while working with sick cats. In the US about five human cases are reported every year.
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