Key facts
- Plague is caused by the bacteria Yersinia pestis, zoonotic bacteria usually found in small mammals and their fleas.
- People infected with Y. pestis often develop symptoms after an incubation period of one to seven days.
- Bubonic plague is the most common clinical form of plague. Bubonic plague is characterized by painful swollen lymph nodes or 'buboes'. Less common, but severe forms include septicaemic and pneumonic plague.
- Plague is transmitted between animals and humans by the bite of infected fleas, direct contact with infected tissues, and inhalation of infected respiratory particles.
- Plague can be a very severe disease in people, with a case-fatality ratio of 30% to 60% for the bubonic type, and it is always fatal for the pneumonic and septicaemic kinds when left untreated.
- Antibiotic treatment is effective against plague bacteria, so early diagnosis and treatment can save lives.
- Human plague remains endemic in several countries. During 2019–2025, most cases reported to WHO occurred in the Democratic Republic of the Congo and Madagascar.
Overview
Plague is an infectious disease caused by the bacteria Yersinia pestis, usually found in small mammals and their fleas. It is transmitted between animals through fleas. Humans can be infected through:
- the bite of infected vector fleas
- unprotected contact with infectious bodily fluids or contaminated materials
- the inhalation of respiratory particles from a patient with pneumonic plague.
Plague is a very severe disease in people, particularly when the bacteria spreads through the bloodstream (septicaemic plague) or infects the lungs (pneumonic plague). It has a case-fatality ratio of 30% to 100% if left untreated. The pneumonic and septicaemic forms are invariably fatal unless treated early.
Pneumonic plague is especially contagious and can trigger severe epidemics through person-to-person transmission via respiratory particles.
Historically, plague was responsible for widespread pandemics with high mortality. It was known as the "Black Death" during the fourteenth century, causing more than 50 million deaths in Europe. Nowadays, plague is easily treated with antibiotics and the use of standard precautions to prevent acquiring infection.
Types of plague
There are three main clinical forms of plague infection: bubonic, pneumonic and septicaemic.
- Bubonic plague is the most common form of plague and is caused by the bite of an infected flea. Plague bacillus, Y. pestis, enters at the bite and travels through the lymphatic system to the nearest lymph node where it replicates itself. The lymph node then becomes inflamed, tense and painful, and is called a ‘bubo’. At advanced stages of the infection, the inflamed lymph nodes can turn into open sores filled with pus. Human-to-human transmission of bubonic plague is rare. Bubonic plague can advance and spread to the lungs, causing pneumonic plague, or to the bloodstream, causing septicaemic plague.
- Pneumonic plague, or lung-based plague, is the most virulent form of plague. Incubation can be as short as 24 hours. Any person with pneumonic plague may transmit the disease via respiratory particles to other humans. Untreated pneumonic plague, if not diagnosed and treated early, can be fatal. However, recovery rates are high if detected and treated in time (within 24 hours of onset of symptoms).
- Septicaemic plague occurs when Y. pestis multiplies rapidly in the bloodstream. It can develop from any other form of plague. It can lead to severe complications, including organ failure, tissue damage, respiratory distress and bleeding under the skin. Septicaemic plague is fatal if left untreated.
Signs and symptoms
Signs and symptoms can differ by the type of plague.
- Bubonic plague symptoms typically include sudden onset fever, chills, head and body aches and weakness, vomiting and nausea. Painful and inflamed lymph nodes can also appear during bubonic plague.
- Symptoms of the pneumonic form appear quickly after infection (sometimes less than 24 hours) after infection. They include severe respiratory symptoms such as shortness of breath and coughing, often with blood-tainted sputum.
- Symptoms of septicaemic plague include bleeding under the skin and, in some cases, a bluish discoloration of the skin (cyanosis) and tissue death (necrosis) of the extremities. The disease can also lead to multiple organ failure and severe breathing difficulties.
Where is plague found?
As an animal disease, plague is found on all continents, except Oceania. There is a risk of human plague wherever the presence of plague natural foci (the bacteria, an animal reservoir and a vector) and human population co-exist.
Figure 1: Countries reporting human plague cases to WHO, 2019–2025. Countries shown as not reporting may still have plague foci.
Plague epidemics have occurred in Africa, Asia and South America, but since the 1990s, most human cases have been reported in Africa, particularly in the Democratic Republic of the Congo and Madagascar. In Madagascar cases of bubonic plague are recorded nearly every year, during the epidemic season (between September and April).
Diagnosing plague
Confirmation of plague requires lab testing. The best practice is to identify Y. pestis from a sample of pus from a bubo, blood or sputum. A specific Y. pestis antigen can be detected by different techniques. One of them is a laboratory validated rapid dipstick test now widely used in Africa and South America with the support of WHO.
Treatment
Untreated pneumonic plague can be rapidly fatal, so early diagnosis and treatment are essential for survival and reduction of complications. Antibiotics and supportive therapy are effective against plague if patients are diagnosed in time. Pneumonic plague can be fatal within 18 to 24 hours of disease onset if left untreated, but common antibiotics for enterobacteria (gram negative rods) can effectively cure the disease if they are delivered early.
Prevention
Preventive measures include informing people when zoonotic plague is present in their environment and advising them to take precautions against flea bites and to not handle animal carcasses. Generally, people should be advised to avoid direct contact with infected body fluids and tissues. Precautions should be taken when handling potentially infected patients and collecting specimens.
Vaccination
WHO does not recommend vaccination, except for high-risk groups (such as laboratory personnel who are constantly exposed to the risk of contamination, and health-care workers).
Managing plague outbreaks
- Find and stop the source of infection. Identify the most likely source of infection in the area where the human case(s) was exposed, typically looking for areas with clusters of small animal deaths. Implement appropriate infection, prevention and control procedures. Vector (flea) control should be carried out before rodent control as killing rodents before fleas will cause the fleas to jump to new hosts.
- Protect health workers. Inform and train them on infection prevention and control. Workers in direct contact with pneumonic plague patients must wear standard personal protective equipment and receive antibiotics (chemoprophylaxis) for a duration of seven days or at least as long as they are exposed to infected patients.
- Ensure correct treatment. Verify that patients are being given appropriate antibiotic treatment and that there are adequate local supplies of antibiotics.
- Isolate patients with pneumonic plague. Patients should be isolated so as not to infect others. Providing masks for pneumonic patients can reduce spread.
- Surveillance. Identify and monitor close contacts of pneumonic plague patients and give them a seven-day chemoprophylaxis. Chemoprophylaxis should also be given to household members of bubonic plague patients.
- Obtain specimens. Specimens should be carefully collected using appropriate infection, prevention and control procedures and sent to labs for testing.
- Disinfection. Routine hand-washing with soap and water or use of alcohol hand rub is recommended. Larger areas can be disinfected using 10% of diluted household bleach (made fresh daily).
- Ensure safe burial practices. Spraying of face/chest area of suspected pneumonic plague deaths should be discouraged. The area should be covered with a disinfectant-soaked cloth or absorbent material.
Plague surveillance and control
Plague surveillance and control requires investigating the animal and flea species in the region that carry the disease and developing environmental management programmes to understand the natural zoonosis of the disease cycle and limit spread. Regular monitoring of animal populations where plague is known to occur, combined with rapid action when animal outbreaks are detected, has helped reduce the number of plague outbreaks in humans. An informed and vigilant health workforce, supported by engaged communities, is critical to the timely detection and effective control of plague outbreaks. Together, they facilitate early diagnosis and treatment, identify risk factors, strengthen human and animal surveillance, and ensure rapid reporting to public health authorities.
WHO response
WHO aims to prevent plague outbreaks by maintaining surveillance and supporting at-risk countries to prepare. As the type of animal reservoir differs according to the region and influences the risk and conditions of human transmission, WHO has developed specific guidelines for the Indian sub-continent, South America and sub-Saharan Africa.
WHO works with ministries of health to support countries facing outbreaks for field control activities.
