An infection caused by the bacterium Yersinia pestis, which lives in wild rodents and is carried between them by fleas. People are usually infected by a flea bite or by handling an infected animal. It is the same disease that caused the Black Death in the 14th century.

What is the difference between bubonic and pneumonic plague?

They are the same infection in different parts of the body. Bubonic plague, the most common form, settles in a lymph node and produces a painful swelling called a bubo. Pneumonic plague infects the lungs. It is rarer, progresses faster, and is the form that spreads between people.

Can plague be treated?

Yes. Several common families of antibiotics work against it, though some treated patients still die. Timing matters most for the lung form: the Centers for Disease Control and Prevention notes that patients were able to recover when treated within the first 24 hours of symptoms.

How easily does pneumonic plague spread?

Less easily than its reputation suggests. It travels in large droplets that fall quickly, and the CDC says person-to-person transmission requires exposure within 6 feet. US data put the average number of people infected by each patient at 1.18.

Was the death in Siberia caused by plague?

That has not been established. Russian authorities say testing showed no evidence of plague. The World Health Organization said on October 7 that it did not yet have the full picture and had asked for the test results. This is general information rather than medical advice.

Plague explained: how it spreads, how it is treated and why a death in Siberia put it back in the news

Plague is a bacterial infection kept alive in wild rodents and their fleas. Antibiotics cut deaths sharply when given early; its lung form, the one that spreads between people, can kill within a day without them. What happened in Siberia is unconfirmed.

A 28-year-old woman who worked at an anti-plague research institute in Irkutsk, in Siberia, died in early October after developing pneumonia, and what caused it has not been established. Russian media suggested she might have had pneumonic plague. Russian authorities say testing showed no evidence of plague, but the research institute has been quarantined.

The World Health Organization (WHO) has asked for more. “We don’t yet have the full picture of this event and we’re not able to conduct a full risk assessment,” its director-general, Tedros Adhanom Ghebreyesus, said on October 7.

Whatever the cause of that death turns out to be, the reports have sent many people looking for facts about a disease they assumed was history. Plague is not history. It is also far more treatable, and far harder to catch, than its name suggests. This explainer sets out what is known.

What plague is and where it lives

Plague is an infection caused by a bacterium, Yersinia pestis. It is first of all a disease of animals. A review of its biology describes it as a zoonotic disease established in stable foci in the Americas, Africa, and Eurasia. Zoonotic means passed from animals to people, and foci are pockets of land where the bacterium persists in wild rodents, their fleas and the soil of their burrows.

People enter the picture by accident. Plague is transmitted between animals and humans by the bite of infected fleas, direct contact with infected tissues, and inhalation of infected respiratory particles, according to the WHO. People infected with Y. pestis often develop symptoms after an incubation period of one to seven days, the gap between infection and the first symptoms.

The geography is wide. As an animal disease, plague is found on all continents, except Oceania. That is why it cannot be wiped out the way smallpox was. “People think the plague is gone, it was something that occurred 500 years ago. It’s not. It’s here amongst us,” Simon Clarke of the University of Reading told The Associated Press.

Its history explains the fear. The WHO puts the toll of the 14th-century pandemic known as the Black Death at more than 50 million deaths in Europe. In Siberia itself the infection is ancient: people in Siberia have been dying of plague for at least 5,500 years, on the evidence of bacterial traces in the teeth of hunter-gatherers.

The three main forms of plague

The same bacterium produces different illnesses depending on where in the body it takes hold.

Form	How it starts	Main signs	Passes between people?
Bubonic	Flea bite or contact with an infected animal	Fever and a painful swollen lymph node	Rarely
Septicemic	Bacteria multiply in the blood	Fever, abdominal pain, bleeding under the skin	No
Pneumonic	Inhaled droplets, or spread from another form to the lungs	Fever, shortness of breath, cough with blood-stained phlegm	Yes, at close range

Bubonic plague is the most common clinical form of plague. After a flea bite the bacteria travel to the nearest lymph node, one of the small glands that filter infection. Within the lymph node, the bacteria multiply, producing a tender swelling, the Centers for Disease Control and Prevention (CDC) explains in its treatment recommendations. The swelling is called a bubo. Human-to-human transmission of bubonic plague is rare, in the WHO’s wording. The CDC goes a step further and counts only the lung form as passing between people.

Septicemic plague is infection of the bloodstream, which can arise on its own or follow either of the others.

Pneumonic plague is infection of the lungs. Primary pneumonic plague develops following inhalation of Y. pestis from an infected animal or human. It can also develop when untreated bubonic plague spreads to the lungs. It is the rarest of the three and the most dangerous.

How pneumonic plague spreads between people

Pneumonic plague is the only clinical form that is transmissible from person to person. That single fact is the reason a suspected case draws international attention.

It is not, however, an airborne infection in the way measles is. Y. pestis, short for Yersinia pestis, the plague bacterium, is transmitted via large respiratory droplets that do not remain suspended in the air for prolonged periods, the CDC writes. No evidence exists for airborne transmission of the kind seen with measles. Person-to-person transmission of Y. pestis requires exposure within 6 feet, and it has mostly been recorded among people caring for a patient or living in the same household.

Researchers have put a number on it. A review of American surveillance data yielded an estimated basic reproductive number (R0, the average number of secondary cases per case) of 1.18 for pneumonic plague in the United States. Each patient, in other words, infected on average just over one other person.

Infectious disease doctors made the same point this week. “So you really need to be in face-to-face contact with that person to get pneumonic plague,” Glenn Wortmann of MedStar Washington Hospital Center told The Associated Press. The respiratory droplets don’t travel very far, maybe 3 to 6 feet, said Dave Wagner of Northern Arizona University.

Crowding changes the arithmetic, as a 2017 outbreak in Madagascar, described below, showed. Adelaide Miarinjara, a medical entomologist at Emory University, meaning a scientist who studies disease-carrying insects, was in Madagascar at the time and put the risk more strongly. “It transmits pretty easily between people because it spreads through droplets, almost like COVID spreads,” she told NPR. That comparison is looser than the CDC’s account of droplets that fall within a few feet.

How deadly plague is, with and without treatment

Before antibiotics, plague was among the most lethal infections known. In that era fatality rates were high, between 66% and 93%, for all clinical forms.

Antibiotics transformed the outlook. Left untreated, the fatality rate of bubonic plague is 66%; however, with antimicrobial treatment, the fatality rate decreases substantially to 13%. Antimicrobial is the broader term for drugs that kill microbes.

The lung form leaves less time. Pneumonic plague can be fatal within 18 to 24 hours of disease onset if left untreated, the WHO states. The CDC describes the other side of that deadline: patients with pneumonic plague, previously considered incurable, were able to recover when treated within the first 24 hours of symptom onset.

A recent American case shows how fast it can move. In July 2025, a man in Arizona died of pneumonic plague after he was exposed to sick cats, NBC News reported. He died within eight hours of reaching a hospital. “The key here is early diagnosis. If people aren’t expecting it or don’t seek treatment, they can die,” Miarinjara said.

How plague is treated and prevented

The drugs are ordinary ones. The CDC traces the sequence: after the first antibiotics in the 1930s, the availability of aminoglycosides, followed by chloramphenicol, tetracyclines, and fluoroquinolones, offered additional options for plague treatment. These are families of antibiotics in everyday hospital use. The agency’s 2021 recommendations, published in the CDC’s own journal, MMWR Recommendations and Reports, moved one widely available drug up the list. It listed ciprofloxacin as a first-line (preferred) agent for treatment of plague rather than an alternative option because more data are now available to support its use.

Resistance has not been a practical problem so far. In the Madagascar epidemic, all 50 isolated Yersinia pestis strains were susceptible to the tested antibiotics.

People exposed to a patient can be given antibiotics before they fall ill, an approach called postexposure prophylaxis. The CDC says it is to be considered for persons who had close (<6 feet), sustained contact with a patient with pneumonic plague and were not wearing protective equipment, and for laboratory workers accidentally exposed. The course lasts seven days.

Vaccines play almost no part. WHO does not recommend vaccination, except for high-risk groups such as laboratory staff. There’s no plague vaccine available in the U.S., The Associated Press noted.

The CDC’s interest in the disease is not only medical. Its report states that the bacterium is recognized as a potential bioterrorism weapon, which is why the United States keeps written plans and drug stockpiles for it.

Where plague still occurs

Yersinia pestis, the bacterium that causes plague, leads to naturally occurring disease in the United States and other regions worldwide. The burden is very uneven.

Most of it falls on two countries. Since the 1990s, most human cases have been reported in Africa, particularly in the Democratic Republic of the Congo and Madagascar. Madagascar accounts for 75% of global plague cases reported to WHO, with an annual incidence of 200-700 suspected cases (mainly bubonic plague), researchers there wrote in 2019.

In the United States the disease is rare and rural. There are an average of about seven human cases per year in the U.S., according to the Centers for Disease Control and Prevention, mostly in the West. A CDC team that went back through the records summarized the characteristics of 1,006 cases of human plague occurring in the United States over 113 years. During 1900-1925, outbreaks were common but were restricted to populous port cities. Over the following four decades the range of the disease expanded rapidly while the number of cases fell. During 1965-2012, sporadic cases occurred annually, primarily in the rural Southwest.

What the 2017 plague epidemic in Madagascar showed

The largest modern test of how pneumonic plague behaves came in Madagascar in 2017, when the infection reached two big cities.

Between August and November, 2414 clinically suspected plague cases were reported, including 1878 (78%) pneumonic plague cases. Most were concentrated in the capital, Antananarivo, and the main seaport, Toamasina.

Two lessons came out of it. The first is that the lung form can spread widely in crowded places when cases are not recognized early. The second is that fear inflates the count. Of the reported pneumonic cases, the investigators found only 23% having one or more positive laboratory tests. That 23% was made up of 386 cases classed as probable and just 32 that met the full standard for a confirmed case. Many of the rest were probably other chest infections, reported and treated as plague during the alarm.

Among the cases that were confirmed, the disease was severe even with treatment available. The case fatality ratio was higher among confirmed cases (eight [25%] of 32 cases) than among those merely suspected, of whom 5% died. A case fatality ratio is the share of patients who die.

What is known about the death in Siberia

Very little is established. The woman died on a Friday at the start of October, according to the World Health Organization, with no cause of death confirmed. Nearly 200 people who came into contact with the woman have been under precautionary medical observation, according to Russian media.

Russia’s health watchdog has reported that it found no dangerous pathogens. The WHO has not seen the underlying results. “What we have been told is that they had no case of plague registered in this area and that no high-threat pathogens were detected,” said Maria Van Kerkhove, who directs the agency’s epidemic and pandemic management department.

It is not known whether the woman had plague at all, and if she did, whether it came from the wild or from bacteria handled in the laboratory. The CDC said it was watching. “At this time, there is no indication of a broader threat to the United States, but CDC and our interagency partners are prepared and will continue to monitor the situation closely,” the agency said.

Specialists asked about the wider risk were unworried. “We will not turn 2026 into the Middle Ages, where the Black Death caused vast amounts of death throughout Europe. That will not happen,” said William Schaffner of Vanderbilt University Medical Center.

Plague survives in wild rodents on every continent except Oceania, responds to common antibiotics, which for the lung form need to start within about a day, and spreads between people in its lung form and at close range, while the cause of the death in Siberia remains unconfirmed.