World health experts are closely following a potential case of pneumonic plague—the most deadly form of the disease. Earlier this week Russian officials reported that an employee at a laboratory at an infectious disease research institute in Irkutsk Oblast in Siberia, had contracted pneumonia and died. The World Health Organization (WHO) is now working closely with the country’s health authorities to monitor and track the incident. But fears over whether the disease could spread are proliferating as officials move to isolate the woman’s contacts and bolster outbreak prevention efforts at local hospitals.

At a press conference on Tuesday, WHO spokesperson Christian Lindmeier said that the woman’s cause of death was still unconfirmed and that lab testing was in progress. But “according to the test results, no pathogens of dangerous infectious diseases were detected among any of the contacts” of the woman, Lindmeier told reporters.

Here’s what you need to know about the potential outbreak and what it means.

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What is plague?

What people commonly refer to as “plague” is an infectious disease caused by the bacterium Yersinia pestis. It is best known for causing the Black Death, which killed tens of millions of people across Europe, as well as Asia and North Africa, in the 1300s.

There are three main forms of plague—bubonic plague, septicemic plague and pneumonic plague. They are all caused by the same bacterium but differ in how the pathogen enters the body and where the infection manifests. Bubonic plague, which is the most common form, is usually spread by the bites of infected fleas or direct contact with infected animals. It affects the lymph nodes, where it causes swollen and painful blue-black sores, called buboes. Septicemic plague is spread in similar ways, but it affects the bloodstream and can lead to a systemic infection. Finally, pneumonic plague occurs when the bacterium infects the lungs—typically through inhalation of infectious droplets rather than flea bites.

How dangerous is plague?

The level of danger depends on what kind of plague is encountered and whether antibiotics are available. The 14th-century Black Death was particularly deadly because there were no antibiotics at the time and people lived in crowded cities, often with poor sanitation.

Today antibiotics are very effective against plague when they are used early in the infection. But even so, Y. pestis is still deadly. Once the disease reaches the bloodstream or lungs, which seems to be potentially what happened in the Russian case, it can quickly turn fatal.

“What we do worry about the most is pneumonic plague,” says Richard Martinello, a professor of infectious disease and pediatrics at the Yale School of Medicine. “People who are sick with pneumonia due to Y. pestis can produce droplets that contain the bacteria, and we know that they could be highly contagious and readily spread the Y. pestis to other people.”

In the case of pneumonic plague, antibiotics have to be given very early on in the infection for them to do any good, Martinello says. If they are given too late, they won’t stop the infection. Pneumonic plague’s mortality rate is almost 100 percent if untreated, but chances of survival are higher with rapid treatment.

Does plague pose a global threat?

Pneumonic plague can spread between people through respiratory droplets, and it can occur anywhere the plague bacteria exist. Martinello notes a recent report from the U.S. Centers for Disease Control and Prevention from earlier this month that describes a fatal case of pneumonic plague that occurred in Coconino County, Arizona, in July 2025. It was Arizona’s first pneumonic plague death since 2007.

But based on the information that is available right now about a single suspected case in Russia, Martinello is less concerned about global spread.

“It’s a concerning time because people are more indoors and they’re exposed to more people and thus exposed to their infectious respiratory droplets,” Martinello says. Still, he adds, “I don’t think we are going to see a great deal of spread by this.”

“If there was substantial and rapid growth, that would make me worried that there may be something else involved or something special about this strain of Yersinia pestis causing the infection—if it is, in fact, Yersinia pestis,” Martinello notes.

The WHO has been closely tracking the incident, and the White House is also monitoring the situation, according to Axios.

“Because this is so contagious and so deadly…, it really relies on that effective public health effort in a timely manner,” Martinello says.