The Bubonic Plague: How it Spreads, Symptoms and Treatment

How the Bubonic Plague Spread and How It Is Treated Today

Cat Flea (Ctenocephalides felis). Credit: John Abbott/Visuals Unlimited, Inc / Getty Images

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 get 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.

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.

  • 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.

  • 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 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 1 to 7 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. Those that have a greater chance of catching plague are:

  • Hunters and trappers
  • Having a house dog or cat can also put people at risk
  • Living in the Southwest 

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 grasslands rodent species can survive.

Many types of animals can be affected by plague, such as:

  • 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, since scientists think that the Plague bacteria may circulate at low rates within populations of certain rodents without causing excessive rodent die-off. 

Diagnosis of Plague

Laboratory tests on sputum, blood, spinal fluid, or infected lymph nodes can show presence of the bacteria. Sudden bloody sputum should be seen and evaluated by a doctor immediately.

Treatment Options

Today, bubonic plague can be effectively treated with certain antibiotics like streptomycin or gentamicin. If left untreated, bubonic plague can become septicemic plague, which is fatal in approximately 50-60% of infected people.

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


Center for Disease Control and Prevention. Plague. Accessed on 6 March 2016.

Vermont Department of Health. Plague. Accessed on 6 March 2016.

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