Plague is an infectious disease caused by the bacterium Yersinia pestis. The organism circulates mainly among wild rodents and their fleas, and reaches people through flea bites, contact with infected tissue, or inhalation of respiratory droplets. Left untreated, it is often fatal, but it responds to antibiotics when diagnosed early. Plague caused some of the deadliest pandemics in recorded history and remains a rare, mostly zoonotic disease today, found in parts of Africa, Asia, and the Americas.
Causative organism
Y. pestis is a bacterium classified as a Gram-negative, non-motile coccobacillus. It has a peptidoglycan cell wall beneath an outer lipopolysaccharide membrane, and it is a facultative anaerobe, able to grow with or without oxygen. Genetically it is closely related to Yersinia pseudotuberculosis, from which it evolved several thousand years ago. Its high lethality is associated with a toxin-producing plasmid.
The bacterium was identified in 1894 by Alexandre Yersin during an epidemic in Hong Kong; Kitasato Shibasaburo isolated it independently in the same year. In 1898 Paul-Louis Simond showed that the disease passes from rodents to humans through flea bites. The species was originally named Pasteurella pestis and was renamed in 1944.
Ecology and transmission
Plague is maintained in an enzootic cycle among wild rodents such as prairie dogs, ground squirrels, and rats, whose fleas carry the bacteria between animals. When an epizootic kills many rodents, hungry fleas turn to other hosts, including people, pets, and livestock, which raises the risk of human infection. Domestic cats and dogs can become ill after eating infected animals or being bitten by infected fleas, and they can sometimes pass the infection to people.
Human infection occurs through three main routes:
- Flea bites, the most common route.
- Direct contact with infected animal tissues, for example while skinning or handling carcasses.
- Inhalation of droplets from a patient with pneumonic plague, which can allow person-to-person spread, though this is uncommon.
Clinical forms
After exposure, symptoms usually appear within one to seven days. The disease takes three main clinical forms.
Bubonic plague is the most common form. It is marked by painful, swollen lymph nodes called buboes, most often in the groin because the legs are the body part most often bitten by fleas. Fever, chills, headache, and weakness typically come first. Without treatment, the infection frequently spreads to the bloodstream, and a smaller share of patients develop pneumonia or meningitis. Estimates of untreated mortality vary across sources, from roughly 30–60% to 40–70%.
Septicaemic plague arises when the bacteria multiply in the blood, either as a primary infection or as a complication of another form. It can cause bleeding under the skin or from mucosal surfaces, shock, and tissue damage from clotting abnormalities.
Pneumonic plague affects the lungs. It has the shortest incubation period and the most rapid course, and it is fatal within about a day of onset if untreated. Patients may produce blood-tinged sputum and develop severe breathing difficulty. This form is the one of greatest concern for public health because it can be transmitted between people.
A mild variant called pestis minor occurs in endemic areas, with lymph node swelling and fever that resolve within about a week.
Diagnosis
Diagnosis is primarily clinical and epidemiological, based on symptoms and possible exposure to rodents, fleas, or infected animals. Laboratory confirmation uses bacterial culture, serological tests, and rapid diagnostic tests recommended in World Health Organization guidance. Under the microscope, Y. pestis shows bipolar staining, particularly with Giemsa stain, which gives it a safety-pin appearance. For suspected pneumonic cases, clinicians must consider other causes of severe pneumonia, including tularemia, anthrax, and inhalation of staphylococcal enterotoxin B.
Treatment and prevention
Plague is treated with antibiotics. Streptomycin or gentamicin are standard first-line drugs, while fluoroquinolones such as ciprofloxacin or levofloxacin, and tetracyclines such as doxycycline, serve as alternatives. Supportive care, including oxygen, intravenous fluids, and respiratory support, is often also necessary. Early treatment is the main determinant of survival. Close contacts of pneumonic patients can receive preventive antibiotics, and those treatments are effective when started within about seven days of exposure.
Patients with pneumonic plague are isolated, and staff use droplet precautions with eye protection. Public health measures aim to reduce people's contact with infected rodents and fleas, including insecticide treatment in affected areas, control of rodent populations, and avoidance of sick or dead animals. A vaccine was developed in the late 19th century by Waldemar Haffkine and was used on a large scale in India into the 1920s, but no vaccine is widely available today.
Epidemiology
The World Health Organization estimates that roughly 1,000 to 3,000 cases are reported worldwide each year. Between 1990 and 2020, nearly 50,000 human cases were reported by 26 countries. Sub-Saharan Africa accounts for the great majority of cases, and the Democratic Republic of the Congo, Madagascar, and Peru have been among the most affected countries in recent years. Madagascar reports the highest number of human cases annually, often between 250 and 500. In Asia, the main active foci are in China, and the disease also persists in parts of Central Asia and the former Soviet Union. Human cases typically occur in rural and agricultural areas rather than large cities.
In the United States, plague is rare. Most human infections occur in rural areas of the western and southwestern states, with an average of about seven cases per year in recent decades.
Recent developments (2026)
Several reports in 2026 illustrate the disease's ongoing presence:
- In April 2026, Apache County in Arizona confirmed a human case; the patient recovered after treatment. This was the first confirmed case in that county since 2015.
- In June 2026, New Mexico reported a fatal human case in Santa Fe County, the state's first human case of the year, following animal infections in the area.
- In October 2026, Madagascar's Ministry of Public Health reported four confirmed and five suspected cases, after a period of increased plague activity that began in late July. Health authorities traced contacts and sprayed insecticides in affected areas.
- In October 2026, Russian authorities reported the death of a laboratory worker at a plague research institute in Irkutsk, Siberia, who was diagnosed with pneumonia of undetermined cause. Testing did not identify plague, and the WHO said no plague case had been registered, while urging Russia to provide more information. The cause of death remained unconfirmed at the time of writing.
Historical pandemics
Plague has caused several major pandemics. The first, known as the Plague of Justinian, spread across the Mediterranean and Europe from the mid-6th century, with death toll estimates varying widely. The Black Death (1346–1353) spread across Europe, Asia, and North Africa and killed roughly a third of Europe's population. The third plague pandemic, which began in southern China in the mid-19th century, spread to many parts of the world and persisted until about 1960. In India alone, an estimated 5.5 to 6 million people died from plague between the late 1890s and about 1917.
Historians and scientists continue to debate how far the Black Death and other medieval outbreaks can be explained by flea-borne transmission, and whether other pathways or other pathogens also played a role.
Use as a biological weapon
Because it can be produced in large quantities and disseminated as an aerosol, Y. pestis is regarded as a potential biological weapon. Aerosol release could cause large numbers of pneumonic cases within one to six days. Plague is subject to international regulations, and suspected cases are notifiable to public health authorities in most countries.
Ongoing significance
Plague is no longer the threat it was in the medieval and early modern periods, largely because antibiotics greatly reduce mortality when treatment starts early. It is nonetheless treated as a re-emerging disease: outbreaks can disappear for years and then resurface, and surveillance of rodent and flea populations remains an important part of prevention. The public health response to plague emphasizes rapid diagnosis, notification, contact tracing, and control of animal reservoirs.
References
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- Plague (Yersinia pestis) Fact Sheetdshs.texas.gov
- How Plague Spreadscdc.gov
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- Plague and Other Yersinia Infections - Infectious Disease - MSD Manual Professional Editionmsdmanuals.com
- Disease information about plagueecdc.europa.eu
- afro.who.intafro.who.int
- graphsearch.epfl.chgraphsearch.epfl.ch
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- Bubonic Plague in Arizona Medium Prioritykdhe.ks.gov
- Arizona: Human plague case reported in Apache Countyoutbreaknewstoday.substack.com
- New Mexico reports first case of human plague in 2026nmhealth.org
- Madagascar reports 4 confirmed plague casesthestar.com.my
- Russia dismisses reports of second plague case, WHO urges formal responsealjazeera.com
- WHO says no plague case registered after Russian lab worker’s deathaljazeera.com
- Pneumonia of undetermined etiology - the Russian Federationwho.int
- List of epidemics and pandemicsen.wikipedia.org