The Bubonic Plague: How it Spreads, Symptoms and Treatment

How the Bubonic Plague Spread and How It Is Treated Today

Cat Flea (Ctenocephalides felis) - a carrier of the bubonic plague
How the bubonic plague spreads and is treated today. Credit: John Abbott/Visuals Unlimited, Inc / Getty Images

You may have learned about the bubonic plague in history class in school, but the truth is that bubonic plague is still with us today. Where does it occur and what are the symptoms? What can be done to treat this disease today?

What is the Bubonic Plague?

Bubonic plague is a potentially fatal infection caused by the bacterium Yersina pestis. Bubonic plague is not usually spread directly from person to person.

Small rodents, such as rats, mice, squirrels, and weasels, carry the infection. These animals have fleas that are infected with the plague bacteria. People may be exposed to the bacteria from flea bites or from direct contact with an infected animal like a cat or dog that touched, bit, or ate an infected mouse or rat. Less commonly, the plague may be spread from person to person through coughing or sneezing.

History of Bubonic Plague

During the Middle Ages, bubonic plague was known as the "Black Death." During that time many people became sick and died of pneumonia when the bubonic plague bacteria infected the lungs, called the "pneumonic plague" which saw the spread of the disease bacteria through coughing and sneezing.

How the Bacterium Spreads and Affects Humans

The Yersinia pestis bacterium can affect people in 3 different ways:

Bubonic plague - With bubonic plague, the bacteria enters the body through a flea bite.

From the site of the bite the bacterium travels to a nearby lymph node, often creating a "buboes." Hallmarks of bubonic plague include:

  • Enlarged and tender lymph glands, or nodes which gave the Bubonic plague its name—the nodes were called "buboes" during the "Black Death"
  • Most common form of plague. 
  • Flea bite exposure may result in primary bubonic plague.

Pneumonic plague - The least common yet most serious form of the plague, pneumonic plague occurs when bacteria in a bubonic plaque spread to the lungs. Once the bacteria is present in the lungs, it can be spread through the air by coughing or sneezing. Hallmarks of pneumonic plague include:

  • When the bacteria infects the lungs, this is known as the pneumonic plague
  • Occurs when a person inhales infectious droplets of the plague bacteria.
  • Most contagious because it can be spread from person-to-person through the air by sneezing or coughing.
  • This is the most serious form of plague.

Septicemic plague - This form of the plague occurs when the bacteria find their way into the bloodstream. This may occur from the spread of a bubonic plaque, or by entry via contaminated materials directly through a crack or sore in the skin. Hallmarks include:

  • This type occurs when the bacteria multiply in the blood
  • Least common form of the plague
  • Flea bite exposure may result in septicemic plague.


The incubation period for both the bubonic plague and pneumonic plague is one to seven days after exposure to the bacteria. Initial symptoms of the illness include:

  • Fever
  • Chills
  • Muscle aches
  • Headache
  • Nausea
  • Vomiting
  • Diarrhea
  • Abdominal pain
  • Extreme exhaustion.
  • Swollen and tender lymph nodes near the infected flea bite (bubonic plague)
  • Cough, bloody sputum, which is a mixture of saliva and phlegm, and difficulty breathing (pneumonic plague)

Does the Bubonic Plague Still Exist Today?

Though the plague is extremely rare in the U.S., the bacterium still exists. About 10-20 cases are reported each year in the U.S., mainly in rural areas of the Southwest. In 2015, 15 cases of bubonic were reported and four of these were fatal. Those that have a greater chance of catching plague are:

  • Hunters and trappers
  • Those living in the Southwest, especially New Mexico, Arizona, and Colorado
  • Those who have a house dog or cat

Since the plague bacteria can cycle between rats and their fleas, the last urban outbreak of rat-associated plague in the U.S. occurred in Los Angeles in 1924-1925. After that time the plague has occurred only in rural and semi-rural areas of the western U.S., primarily in semi-dry forested areas and where grasslands rodent species can survive.

How Common is the Plague?

The World Health Organization receives 1,000 to 2,000 reports of the plague each year. In contrast, between 1900 and 2012, only a little over 1,000 cases were reported in the United States. The most common countries in which plague occurs are Madagascar, the Democratic Republic of Congo, and Peru.

In contrast to the plague, it's thought that the flu virus kills between 250,000 and 300,000 people each year around the world, and infection which is preventable through immunization.

What Animals May be Affected by the Plague?

Animals that may be affected by the plague include:

  • Rock squirrels
  • Wood rats
  • Ground squirrels
  • Prairie dogs
  • Chipmunks
  • Mice
  • Voles
  • Rabbits
  • Wild animals can become infected by eating other infected animals.

These infected animals and their fleas work as long-term reservoirs for the bacteria. Scientists believe that the plague bacteria may persist by circulating at low rates within the populations of certain rodents. If excessive rodent die-off does not occur, the plague may remain present for an extended period of time.,

Diagnosis of Plague

Laboratory tests on sputum, blood, spinal fluid, or infected lymph nodes can show presence of the bacteria. Sudden bloody sputum (coughing up blood) is one symptom and should be evaluated by a doctor immediately (both due to the possibility of plague and other reasons.)

Treatment Options

Today, bubonic plague can be effectively treated with certain antibiotics like streptomycin or gentamicin

What Happens of Plague is Not Treated?

If left untreated, bubonic plague can become septicemic plague, which is fatal in approximately 60 percent of infected people.

If not treated quickly (right after the onset of symptoms), pneumonic plague is almost always fatal.

Prevention of Bubonic Plague

Prevention of bubonic plague may occur on several different levels. Avoiding rodents through careful management of garbage and waste is important to reduce exposure to rodents that may carry fleas. Decreasing exposure to the fleas themselves can be done by preventing house cats and dogs from contracting fleas, and wearing insect repellent when out of doors in areas where plague could be a risk. Wearing shirts with long sleeves and long pants is also recommended. Research is in progress trying to develop a vaccine which is effective against plague.

Bottom Line on the Plague in Today's World

The plague is but one of several infectious diseases that were once rapidly fatal and devastating to a population, but can now be treated. That said, prompt treatment is critical for many of these, and deaths still occur due to delays in diagnosis and treatment. Regardless of what infectious organism may be the culprit, it is important to seek immediate medical care if you should develop an unexplained fever, rash, cough, or any other symptoms which concern you. Learn more about emerging infections and disease outbreaks.


Centers for Disease Control and Prevention. Plague in the United States. Updated 11/29/16.

Centers for Disease Control and Prevention. What is Plague? Updated 09/14/15.

Verma, S., and U. Tuteja. Plague Vaccine Development: Current Research and Future Trends. Frontiers in Immunology. 2016. 7:602.

World Health Organization. Plague. Updated 09/16.

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