Open Outbreak

Plague

Also known as: Yersinia pestis, bubonic plague, pneumonic plague, septicaemic plague

Plague is an infection caused by the bacterium Yersinia pestis. It is mainly a disease of wild rodents and their fleas, so most human cases start when a person is bitten by an infected flea or has direct contact with infected animals or materials. A more dangerous form, pneumonic plague, affects the lungs and can spread from person to person through respiratory droplets. WHO says plague is severe but treatable, and that early diagnosis and prompt antibiotics are essential for survival. In Madagascar, where plague is endemic, WHO reports that 200 to 400 cases are usually notified each year, mostly bubonic. Plague still has stable natural foci in parts of Africa, the Americas, and Eurasia, so it has not disappeared. There is no widely used licensed plague vaccine for the general public in current practice from the sources found here. Treatment relies on antibiotics, supportive care, and rapid public-health action such as finding contacts and giving preventive antibiotics when needed.

Key facts

TransmissionUsually from bites of infected fleas linked to small mammals and rodents; also by direct contact with infected animals or materials; pneumonic plague can spread by inhaling respiratory droplets from an infected person.
IncubationOften 1 to 7 days overall; pneumonic plague can begin after a shorter incubation, sometimes about 1 to 4 days.
Case fatalityWithout prompt treatment, pneumonic plague is described by WHO as almost always fatal. In the 2021 Madagascar outbreak, confirmed cases had a 37% case fatality ratio (8/22). Older reviews describe high mortality without treatment and much lower mortality with effective antibiotics, but exact figures vary by form and setting.
Reproduction numberunknown
VaccineNo widely used licensed human plague vaccine was identified in the sources found here; experimental and past vaccines are discussed in the literature.
TreatmentPrompt antibiotics are the main treatment; WHO and CDC-linked literature emphasize early diagnosis, treatment, supportive care, and antibiotic prophylaxis for close contacts when indicated.
ReservoirWild rodents and other small mammals, with fleas acting as vectors.
Endemic regionsNatural plague foci persist in parts of Africa, the Americas, and Eurasia; Madagascar is specifically described by WHO as endemic.

Transmission

Plague is usually a zoonosis, meaning people catch it from animals rather than from other people. The main cycle is between small mammals, especially rodents, and their fleas. Humans can become infected after a bite from an infected flea. They can also be infected through direct contact with infected animals, tissues, or contaminated materials. A lung infection called pneumonic plague is the exception because it can spread between people through respiratory droplets after close contact. WHO notes that plague has three main forms: bubonic, septicaemic, and pneumonic, and the route of infection helps determine which form develops. The natural-history review says stable plague foci remain in parts of Africa, the Americas, and Eurasia because the bacterium persists in animal hosts and flea vectors. That is why plague still appears in outbreaks even though it is much rarer than in past centuries.

Symptoms and severity

Symptoms depend on the form of plague. In the 2021 Madagascar outbreak, WHO reported fever, headache, weakness, shortness of breath, chest pain, and cough among suspected pneumonic cases. Bubonic plague commonly causes a painful swollen lymph node, often called a bubo. Septicaemic plague is a bloodstream infection and can progress quickly. The disease can be very severe. WHO says pneumonic plague is very severe and almost always fatal if it is not treated promptly. WHO also warns that untreated bubonic plague can progress to pneumonic plague after the bacteria spread to the lungs. Incubation is not reported in the WHO outbreak page used here, but the literature found in this session describes plague as an acute infection with onset after a short incubation, often within days. The exact risk of death varies by form, speed of diagnosis, and access to antibiotics, so the best-supported message is that delay is dangerous.

Treatment and vaccines

Plague can be treated, but timing matters. WHO says early diagnosis and treatment are essential for survival and for reducing complications. During the 2021 Madagascar outbreak, reported cases were managed in health facilities, and more than 1,000 close contacts were identified, followed up, and given chemoprophylaxis with cotrimoxazole or doxycycline. A 2021 recommendations paper found in the literature confirms that antibiotics are the basis of treatment, pre-exposure prophylaxis, and post-exposure prophylaxis. Supportive care is also important, especially for severe illness affecting the lungs or bloodstream. On vaccines, the sources found here do not show a widely used licensed plague vaccine in current public-health use. The literature includes research on recombinant vaccine candidates and immunogenicity studies, but that is not the same as a broadly available routine vaccine program. For a worried reader, the practical point is simple: plague is a medical emergency, and fast antibiotic treatment is the key tool.

Where it occurs

Plague has not vanished. A recent natural-history review says stable natural foci remain in the Americas, Africa, and Eurasia. WHO specifically describes Madagascar as endemic and says cases are reported there every year in both bubonic and pneumonic forms. In that country, transmission is seasonal, with the more favorable period usually running from September to April, mainly in the central highlands above 700 meters. WHO also describes plague as endemic in Rethy health zone in the Democratic Republic of the Congo. These reports show that modern plague is usually a disease of defined ecological areas where the bacterium circulates among animals and fleas, with occasional spillover into people. That means cases are not evenly spread across the world. Risk is highest in places with established animal reservoirs and vectors, and in people exposed to rodents, fleas, or infected animals. Travel-associated alerts can still happen when cases move from an endemic area to another country.

Recent history

On this site, plague has 24 archived events since 1996, plus one active record: “Irkutsk Oblast plague scare.” The WHO outbreak archive in this session shows repeated notices from Madagascar, the Democratic Republic of the Congo, and occasional reports or alerts linked to Seychelles, Peru, China, Algeria, Malawi, India, Zambia, Namibia, Uganda, and Mozambique. One of the biggest recent events was the 2017 Madagascar epidemic, which the archive summary lists at 2,417 cases and 209 deaths. WHO later described the 2021 Madagascar outbreak as smaller but still serious, with 22 confirmed cases and 8 deaths, including mostly pneumonic cases. WHO’s assessment is that plague in Madagascar is seasonal and expected to recur, so the issue is not only one-off outbreaks but repeated flare-ups in endemic settings. Historically, plague also caused the Black Death and the Third Pandemic, but current public-health concern is about these smaller modern outbreaks and whether they spread beyond their usual ecology.

What to watch

The biggest warning sign would be more sustained human-to-human spread of pneumonic plague, especially outside the usual endemic areas. WHO stresses that pneumonic plague is the form that spreads through respiratory droplets and is very severe if treatment is delayed. Another concerning signal would be spread into new regions or cities, as WHO highlighted when describing the unusual urban character of Madagascar’s 2017 epidemic. Watch also for signs that animal surveillance is worsening, such as higher rat infection rates or flea indices in endemic zones, because that can raise the chance of human cases. Antibiotic resistance would be especially important, since treatment and preventive antibiotics are the main tools identified in the sources found here. Finally, limits on outbreak response capacity matter: delayed diagnosis, weak contact tracing, poor access to antibiotics, or shortages of protective supplies can all make a local outbreak harder to control. No established reproduction number was identified in the sources used here, so surveillance signals matter more than a single headline number.

Related outbreaks

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