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    Yersinia pestis (plague bacterium)

    Also known as Y. pestis, plague bacterium

    Yersinia pestis is the bacterium that causes plague. It circulates among wild rodents and their fleas and infects people mainly through flea bites.[1][2][3] It caused the Black Death, and ten countries still reported 423 plague deaths to WHO in 2019-2025; antibiotics cure it if given early.[4][5][6]

    Editor reviewedStrict sourcingUpdated WikiHealth and medicine, Life sciences
    Key facts

    Yersinia pestis is the bacterium that causes plague. It is zoonotic, meaning it passes from animals to people, and it is usually found in small mammals and their fleas.[1] It matters now because the October 2026 death of a worker at an Irkutsk anti-plague institute, which studies the organism, prompted questions about whether plague was involved.[7][8] This page is not medical advice; talk to a clinician about any health concern.

    Life cycle in nature

    Y. pestis is transmitted by fleas and cycles naturally among wild rodents. Occasionally it spreads widely among animals in an epizootic, and people are usually at greatest risk during or shortly after one.[2] Between epizootics, some rodent populations carry it at low levels without large die-offs, acting as long-term reservoirs. In the western US, affected species include rock squirrels, wood rats, ground squirrels and prairie dogs.[9] Cats are particularly susceptible. They can be infected by eating rodents and can pass infectious droplets to owners and vets.[10]

    How it causes disease

    The bacterium reaches people through flea bites, contact with infected tissue, or inhaled respiratory particles.[11] After a flea bite it multiplies in a lymph node near the bite, causing bubonic plague, and without treatment it can spread further.[12] Infection of the bloodstream is septicaemic plague, and infection of the lungs is pneumonic plague.[13][14] Only the pneumonic form passes between people.[15] Untreated pneumonic and septicaemic plague are always fatal, and untreated bubonic plague kills 30% to 60% of patients.[16]

    Detection and treatment

    Laboratories confirm plague by identifying Y. pestis in pus from a bubo, blood or sputum. A rapid dipstick antigen test is used widely in Africa and South America.[17] The bacterium responds to common antibiotics if they are given early.[6] CDC guidance lists gentamicin and fluoroquinolones as first-line treatments in the US.[18] Some of these approvals rest on animal studies, with limited clinical data in human plague.[19]

    Where it is found

    As an animal disease, plague occurs on every continent except Oceania. In 2019-2025 most human cases reported to WHO were in the Democratic Republic of the Congo and Madagascar.[20] In that period ten countries reported nearly 4,000 cases and 423 deaths.[5] The US averages about five human cases a year.[21] For the disease in more depth, see plague explained.

    History

    Y. pestis caused the fourteenth-century “Black Death”, which killed more than 50 million people in Europe.[4] It reached the US in 1900 on rat-infested steamships. The last urban outbreak in the US was in Los Angeles in 1924-1925, after which the bacterium became established in rural rodents in the West.[22]

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    Questions readers ask

    What kind of organism is Yersinia pestis?

    A zoonotic bacterium, meaning one that passes from animals to people. It is usually found in small mammals and their fleas.[1]

    Which form of plague infection spreads between people?

    Only pneumonic plague, through inhaled droplets, and usually only with direct and close contact.[15]

    Does Yersinia pestis respond to antibiotics?

    Yes. WHO says antibiotic treatment is effective and early treatment saves lives. CDC guidance lists gentamicin and fluoroquinolones as first-line drugs in the US.[6][18]

    Is Yersinia pestis found in Siberia?

    Yes. WHO says the Irkutsk institute at the centre of the October 2026 case is in a part of Siberia where plague occurs naturally in wild animals.[7]

    Sources

    · 24 claims from 7 publishers

    Each numbered claim is a statement we checked against the sources listed with it. Tap a number in the text to see its sources in place.

    1. [1]

      Plague is caused by the bacterium Yersinia pestis, which is usually found in small mammals and their fleas. confirmedas of 2026-10-11

      • Plague (fact sheet) · World Health Organization (retrieved 2026-10-11)
      • About Plague · US Centers for Disease Control and Prevention · 2026-08-24 (retrieved 2026-10-11)
    2. [2]

      Y. pestis persists in cycles among wild rodents and their fleas, and people are usually at greater risk during or shortly after an outbreak among animals (an epizootic). confirmedas of 2026-10-11

      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
    3. [3]

      The CDC says plague bacteria are most often transmitted by the bite of an infected flea, and people can also be infected by handling infected animals or through sick pets, especially cats. confirmedas of 2026-10-11

      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
    4. [4]

      Plague was known as the "Black Death" in the fourteenth century, when it caused more than 50 million deaths in Europe. confirmedas of 2026-10-11

    5. [5]

      Ten countries reported nearly 4,000 human plague cases and 423 deaths to WHO between 2019 and 2025. confirmedas of 2026-10-07

    6. [6]

      Untreated pneumonic plague can be fatal within 18 to 24 hours of onset, but common antibiotics can cure it if given early; WHO says recovery rates are high when treatment starts within 24 hours of symptoms. confirmedas of 2026-10-11

    7. [7]

      The institute studies dangerous pathogens, including plague, in a part of Siberia where plague occurs naturally in wild animals; it opened in the 1930s. confirmedas of 2026-10-08

    8. [8]

      As of 8 October 2026 Russian authorities had not said definitively whether plague could have caused the death, and WHO said it did not know the causative agent. confirmedas of 2026-10-08

    9. [9]

      Plague bacteria are thought to circulate at low rates in some rodent populations without causing large die-offs, so infected animals and their fleas act as long-term reservoirs; in the western US, affected species include rock squirrels, wood rats, ground squirrels and prairie dogs. confirmedas of 2026-10-11

      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
    10. [10]

      Cats are particularly susceptible to plague, can be infected by eating infected rodents, and can pass infectious droplets to owners or veterinarians. confirmedas of 2026-10-11

      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
    11. [11]

      Plague spreads between animals and humans through the bites of infected fleas, direct contact with infected tissues and inhalation of infected respiratory particles. confirmedas of 2026-10-11

    12. [12]

      In bubonic plague the bacteria multiply in a lymph node near where they entered the body and, without treatment, can spread to other parts of the body. confirmedas of 2026-10-11

    13. [13]

      Bubonic plague, marked by painful swollen lymph nodes called buboes, is the most common form; septicaemic and pneumonic plague are less common but severe. confirmedas of 2026-10-11

    14. [14]

      Bubonic plague typically causes sudden fever, chills, head and body aches, weakness, vomiting and nausea with painful lymph nodes; pneumonic plague causes severe respiratory symptoms such as shortness of breath and coughing, often with blood-tainted sputum. confirmedas of 2026-10-11

    15. [15]

      Inhaled droplets from a person or animal with pneumonic plague are the only way plague spreads between people, and this typically requires direct and close contact; the CDC says person-to-person spread has not been documented in the US since 1924. confirmedas of 2026-10-11

      • How Plague Spreads · US Centers for Disease Control and Prevention (retrieved 2026-10-11)
    16. [16]

      WHO puts plague's case-fatality ratio at 30% to 60% for the bubonic form and says pneumonic and septicaemic plague are always fatal when left untreated. confirmedas of 2026-10-11

    17. [17]

      Confirming plague requires laboratory testing, ideally identifying Y. pestis in pus from a bubo, blood or sputum; a rapid dipstick antigen test is widely used in Africa and South America. confirmedas of 2026-10-11

    18. [18]

      CDC clinical guidance lists gentamicin and fluoroquinolones as first-line plague treatments in the US, typically for 10 to 14 days, and tells clinicians never to delay treatment while waiting for test results. confirmedas of 2026-10-11

    19. [19]

      CDC notes that some first-line plague antibiotics were approved by the FDA on the basis of animal studies with limited human clinical data, while doxycycline was compared with gentamicin in a randomised trial. confirmedas of 2026-10-11

    20. [20]

      As an animal disease plague is found on every continent except Oceania; since the 1990s most human cases have been reported in Africa, and in 2019-2025 most cases reported to WHO were in the Democratic Republic of the Congo and Madagascar. confirmedas of 2026-10-11

    21. [21]

      The US reports an average of five human plague cases a year (range 0-17), more than 80% of them bubonic, mostly in northern New Mexico and Arizona. confirmedas of 2026-10-07

    22. [22]

      Plague reached the US in 1900 on rat-infested steamships; after the last US urban epidemic, in Los Angeles in 1924-1925, it spread to rural rodents and became entrenched in the western US. confirmedas of 2026-10-07

    23. [23]

      WHO says plague symptoms usually appear after an incubation period of one to seven days; the CDC gives 2 to 8 days for bubonic plague and as little as about one day for pneumonic plague after inhalation. confirmedas of 2026-10-11

    24. [24]

      WHO does not recommend plague vaccination except for high-risk groups such as exposed laboratory staff and health workers, and the CDC says a plague vaccine is no longer available in the United States. confirmedas of 2026-10-11

    Revision history (1)
    1. Page created.

    Created Oct 11, 2026. Last reviewed by an editor on Oct 11, 2026. Next scheduled review: Jan 11, 2027.

    Cite this page

    "Yersinia pestis (plague bacterium)." ContentLora, updated Oct 11, 2026. https://contentlora.com/wiki/yersinia-pestis

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