Plague is a life-threatening infection caused by the organism Yersinia pestis. There are three types of plague. Bubonic plague is the most common type in humans. Infected fleas transmit Y. pestis.

What is it?

Plague is a life-threatening infection caused by the organism Yersinia pestis. There are three types of plague. Bubonic plague is the most common type in humans. Infected fleas transmit Y. pestis primarily among rodents. When an outbreak kills many rodents, infected fleas can jump to other animals and humans, spreading the infection. Improved living conditions and health services have made human outbreaks uncommon, but occasional plague cases occur.

Concern exists about the use of plague as a biological weapon. Plague bacteria could be put into a form that might be sprayed through the air, infecting anyone inhaling it and causing pneumonic plague. This form affects your lungs and can spread from person to person.

Fortunately, when given promptly, antibiotics usually effectively treat plague.


There are three types of plague: bubonic, septicemic and pneumonic. Signs and symptoms vary depending on the type of plague. It's possible to develop more than one type.

Bubonic plague

Signs and symptoms of bubonic plague generally appear within two to eight days of a plague-infected fleabite. After you're bitten, the bacteria travel through your lymphatic system, infecting the first lymph node they reach. The resulting enlarged lymph node (bubo) is usually 1 to 10 centimeters in diameter, swollen, painful and warm to the touch. It can cause so much pain that you can't move the affected part of your body. The bubo usually develops in your groin, but may also appear in your armpit or neck, depending on where the flea bit you.

Signs and symptoms of bubonic plague include:

  • Buboes — swollen, painful, warm lymph nodes
  • Sudden onset of fever and chills
  • Headache
  • Fatigue or malaise
  • Muscle aches

Septicemic plague

Septicemic plague occurs when plague bacteria multiply in your bloodstream. If septicemic plague occurs as a complication of bubonic plague, buboes may be present.

Signs and symptoms include:

  • Fever and chills
  • Abdominal pain, diarrhoea and vomiting
  • Bleeding from your mouth, nose or rectum, or under your skin
  • Shock
  • Blackening and death of tissue (gangrene) in your extremities, most commonly your fingers, toes and nose

Pneumonic plague

Pneumonic plague — which can occur as a complication of another type of plague or by inhaling infectious droplets coughed into the air by a person or animal — is the least common form of plague. But it's also the most rapidly fatal. Early signs and symptoms, which generally occur within a few hours to a few days after inhaling contaminated droplets, include:

  • High fever
  • Weakness
  • Signs of pneumonia, including chest pain, difficulty breathing and a cough with bloody sputum
  • Nausea and vomiting

Pneumonic plague progresses rapidly and may cause respiratory failure and shock within two days of infection. If antibiotic treatment isn't initiated within a day after signs and symptoms first appear, the infection is likely to be fatal. 


Plague has afflicted humans throughout history. The cause of plague, the Yersinia pestis bacterium, was discovered in 1894 by Alexandre Yersin. Soon after, scientists realised that fleas transmitted the bacteria. Three types of plague exist.

Bubonic plague

Bubonic plague is the most common type of plague in humans. It's usually caused by a bite from an infected flea. Y. pestis bacteria primarily infect animals such as squirrels, rabbits and prairie dogs. You may become infected by a fleabite if you're in close contact with such animals. The bacteria can also enter through a cut in your skin if you handle these animals. Domestic cats that come into contact with infected animals also may transmit the infection to humans.

Septicemic plague

Septicemic plague occurs when plague bacteria multiply in your bloodstream. This happens when bacteria transmitted by a fleabite enter directly into your bloodstream, or as a complication of bubonic or pneumonic plague.

Pneumonic plague

Secondary pneumonic plague can develop if you're infected with another type of plague. In this case, the infection spreads to your lungs, causing a virulent pneumonia that can often be fatal. Primary pneumonic plague can occur when you inhale droplets coughed into the air by a person or animal with pneumonic plague.

Plague as a bioterrorism agent

Plague is also one of a number of potential agents of bioterrorism, along with anthrax, smallpox, botulism, tularemia and nerve gases. It's possible that plague bacteria could be turned into an aerosol and then be spread over large populations. 

Risk factors

Certain factors may put you at greater risk of plague:

  • Location. Naturally occurring plague outbreaks are most common in rural areas and in urban areas characterized by overcrowding, poor sanitation and a high rat population. Outbreaks can happen at any time of year. Plague is present on most continents other than Australia. The greatest number of human plague infections occurs in countries such as Madagascar, Tanzania and the Democratic Republic of Congo. However, the largest concentration of infected animals is in the United States — particularly in New Mexico, Arizona and Colorado — and in the former Soviet Union.
  • Time of year. Most plague infections occur from May to October. During these months, infected rodents and fleas are most active and people are more often outside and exposed to them.
  • Contact with certain animals. Rats, squirrels, rabbits and prairie dogs are common sources of infection. Domestic cats also may become infected by such animals and pose a transmission risk to humans. The disease usually spreads through fleabites, but you can also contract plague after being exposed to an infected animal that may have coughed infectious droplets into the air or through a break in your skin after handling an animal with plague. Groups at increased risk include veterinarians, cat owners, hunters, campers and hikers in areas with recent plague outbreaks among animals.


Complications of plague may include:

  • Gangrene of your fingers and toes resulting from clots in the small blood vessels of your extremities
  • Severe shock
  • Sudden, severe lung failure (acute respiratory distress syndrome, or ARDS)
  • Bloodstream infection (septicemia)
  • Inflammation of the membranes and fluid surrounding your brain and spinal cord (meningitis)
  • Death

With prompt treatment, the overall fatality rate from plague is less than 15 percent. Without treatment, mortality rates can be as high as 60 percent for bubonic plague and 100 percent for pneumonic plague. Death can occur within days after symptoms first appear if treatment doesn't begin promptly.


Your doctor may suspect plague if you live in a high-risk region or report a suspicious exposure. With the exception of a visible bubo, signs and symptoms often mimic other, much more common infectious diseases.

Medical history

You'll likely be asked to describe the type and severity of your symptoms and tell your doctor about your recent history, including whether you've been exposed to sick animals or travelled to areas with plague.

Bubo and respiratory fluid examination

If plague is suspected, your doctor may confirm the diagnosis through microscopic examination of fluid extracted from your bubo, bronchi or trachea. Needle aspiration is used to obtain fluid from your bubo. Fluid is extracted from your airways using endoscopy. In this procedure, a thin, flexible tube is inserted through your nose or mouth and down your throat. A suction device is sent down the tube to extract a fluid sample from your airways.

Blood test

Your doctor may also test blood drawn from your veins to diagnose plague. Y. pestis bacteria generally are present in your bloodstream only if you have septicemic plague.