What is pneumonic plague and how dangerous is it?

The bacteria Yersinia pestis causes plague, including the most common, bubonic plague, which typically results from the bite of an infected flea. Pneumonic plague is the most serious form of the illness. Centers for Disease Control and Prevention/Getty Images via CNN Newsource

A 28-year-old lab researcher at a Russian institute dedicated to studying plague and dangerous pathogens died of pneumonia of an “unknown” origin.

The Irkutsk Research Anti-Plague Institute operates in Siberia, where several other people are quarantined. Rospotrebnadzor, the Russian agency that oversees infectious disease, has downplayed the link between the researcher’s death and her work, but the case has prompted questions about the type of plague that causes pneumonia.

The U.S. State Department and Centers for Disease Control and Prevention said they are monitoring the situation closely. The World Health Organization said Monday it’s in contact with Russian health authorities to verify what happened, what testing is underway and any potential international public health implications.

“Based on unofficial information available, the public health risk to the general population appears to be low,” a WHO spokesperson said, adding that its risk assessment would be updated once more information was available.

What is pneumonic plague?

Pneumonic plague is a rare, deadly lung infection caused by the bacterium Yersinia pestis. Scientists consider pneumonic plague the most dangerous form of the infection.

There are other forms of plague stemming from Y. pestis that attack different parts of the body.

The most common form, bubonic plague, often develops after a flea bite or when the bacterium 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.

How is pneumonic plague transmitted?

Pneumonic plague is the only form of the bacterial infection that passes directly from person to person. Respiratory particles carry it when a person coughs or sneezes. Droplets from infected animals can also transmit it.

The last documented person-to-person transmission of pneumonic plague in the United States dates back to 1924, during an outbreak in Los Angeles.

What are pneumonic plague symptoms?

With pneumonic plague, particles lodge in the lungs and can trigger severe pneumonia, making breathing difficult. Symptoms can begin within 24 hours of infection.

Other symptoms include fever, chills, headache, body aches, dizziness, weakness, fatigue, nausea, vomiting and diarrhea.

Without treatment, it is nearly always deadly.

How is pneumonic plague treated?

Doctors can treat plague with at least 10 to 14 days of antibiotics. Most commonly they use ciprofloxacin, levofloxacin, moxifloxacin, gentamicin, and streptomycin. Patients that get antibiotics within the first 24 hours of infection have the best chance of surviving.

Doctors may also support someone with supportive care and provide supplemental oxygen, fluids and encouragement to rest.

Is there a vaccine for plague?

Although plague vaccines have been in use since the 19th century, it wasn’t clear they offer protection from pneumonic plague.

The WHO doesn’t recommend vaccination, except for high-risk groups such as laboratory personnel who are constantly exposed to the risk of contamination.

Why does plague still happen today?

Although people may be most familiar with plague from reading the Bible or historical literature, it persists today.

Known as the “Black Death” in the 14th century, the infection killed more than 50 million people in Europe. While antibiotics have made it more manageable, it remains present on every continent except Oceania.

Since the 1990s, most human cases have occurred in Africa, according to the WHO.

The last U.S. death from pneumonic plague occurred in Arizona in July 2025, according to the CDC . The young man worked with sick cats and is believed to have contracted the infection from the animals. No one else exposed to the cats reported getting sick.

How can people avoid getting sick?

WHO recommends people avoid direct contact with those who are sick and urges caution around infected body fluids.

Workers caring for patients with pneumonic plague must wear standard protective equipment and may also take preventive antibiotics.

In case of exposure, health officials will also monitor family members and other close contacts of an infected person and provide them with antibiotics before they become ill. They also recommend routine hand-washing with soap and water.

By Jen Christensen, CNN