Plague (PLAYG) is a potentially serious bacterial infection that is spread to humans by infected fleas carried in the fur of rodents.

What Is Plague?

Plague is a disease caused by the bacterium Yersinia pestis (yer-SIN-e-uh PES-tis). It has been in existence for at least 2,000 years, and in the 21st century is still found in Africa, Asia, South America, and North America.

There are three types of plague: pneumonic (nu-MOnik) plague involves the lungs; bubonic (byoo-BAH-nik) plague, the most common form, involves the body's lymphatic system * ; and septicemic (sep-ti-SE-mik) plague involves the bloodstream and spreads throughout the body. Septicemic plague can occur by itself or along with pneumonic or bubonic plague.

Wild rats and fleas often are associated with plague because they were the primary vectors of the disease during the most devastating outbreaks. Other types of rodents (and their fleas) can carry plague as well, such as prairie dogs, chipmunks, wood rats, and ground squirrels.

How Common Is Plague?

Plague is a serious infectious disease transmitted by the bites of rat fleas. There are three major forms of plague: bubonic, pneumonic, and septicemic.

Plague is a serious infectious disease transmitted by the bites of rat fleas. There are three major forms of plague: bubonic, pneumonic, and septicemic. The illustration represents the three stages of transmission (infection of the flea; flea infecting rodents; flea biting humans).
llustration by Electronic Illustrators Group. © 2016 Cengage Learning®.

How Is Plague Spread?

Plague is transmitted in several ways. The most common of these is from animal to human through the bite of infected fleas. Fleas living on infected animals ingest the animals’ blood and the bacteria in it. They then spread the disease to other animals and humans through their bite, which can result in the bubonic or septicemic form of plague. Bacteria also can enter the body through an open cut or wound after direct contact with infected people or animals.

In addition, humans and animals (such as cats) with plague can spread the disease by releasing tiny drops containing the bacteria from their mouth and nose, which happens in humans when a person coughs, sneezes, or talks. As these droplets enter the air, the smaller ones can float for up to one hour, whereas the larger drops settle on nearby objects. A sneeze or cough can send thousands of infected particles into the air. If inhaled, these droplets can cause the pneumonic form of plague. This way of spreading the disease requires relatively close contact with an infected person or animal.

The first pandemic (a worldwide outbreak of disease) of plague chronicled by historians occurred between 542 and 546 CE, during the reign of the emperor Justinian the Great (482/483–565). The plague followed trade routes to other countries, and the Roman army itself carried plague during war campaigns throughout Asia Minor, western Europe, Italy, and Africa. Outbreaks continued for the next 300 years before the disease finally subsided.

An equally devastating second pandemic erupted nearly 800 years later, as plague once again traveled across trade routes and infected populations throughout Europe. Known as the Black Death, this 14thcentury outbreak killed more than one-third of Europe's population.

During these two pandemics, the cause of plague (the bacterium spread by rats and more importantly, by their infected fleas) was unknown. The spread of disease went unexplained, and many people feared it was a punishment sent by God.


A new case of the plague was identified on August 18, 2015, in California. This patient was the third person identified as having contracted the plague in 2015 after visiting Yosemite National Park. The second patient was also from California, and in addition one person from Georgia contracted the disease.

In response to the second case of the plague in Yosemite, the campgrounds were shut down after determining that two dead squirrels had the plague. The campgrounds remained closed until treated with deltamethrin, a chemical that kills fleas that spread the plague.

One child diagnosed with the plague from Yosemite National Park is still recovering. Unfortunately, a teenager and an adult from Colorado with the disease died. The Centers for Disease Control and Prevention reported that all four victims had the bubonic form of the plague.

To avoid contracting the disease, the CDC advises all campers, hikers, and other visitors to wilderness areas to avoid contact with wild rodents.

What Are the Signs and Symptoms of Plague?

Symptoms typically appear two to six days after infection. Sudden fever, chills, and headache, followed by swollen, painful, hot-to-the-touch lymph nodes * known as buboes (BYOO-boze), are the hallmarks of bubonic plague. Lymph nodes in the groin are most commonly affected. If left untreated, the infection eventually spreads to the bloodstream, causing sepsis * , pneumonia * , or meningitis * .

In septicemic plague, the bacteria multiply in the blood, causing symptoms such as fever, chills, weakness, abdominal * pain, nausea (NAW-zha), and vomiting. As the infection progresses, the blood pressure drops and the blood is unable to clot * normally. The skin looks bruised from uncontrolled bleeding, which is why the disease was called the Black Death.

The pneumonic form of plague takes hold rapidly, with symptoms such as fever, cough, chills, chest pain, bloody sputum * , and headache. It can progress to respiratory failure * and shock * within two to four days.

How Do Doctors Diagnose and Treat Plague?

Determining whether a person was in close contact with animals that can carry plague or has traveled to an area where the plague is known to occur can be crucial to making the diagnosis. Bubonic plague can be identified by the characteristic swollen lymph nodes. A blood culture * * may be done, as well as a culture of a sputum sample to look for Yersinia pestis bacteria.


Plague bacteria are considered to be one of several deadly organisms that could be used in biological warfare. Scientists fear that the bacteria could be aerosolized (AIR-o-suh-lized), or processed into tiny particles that could be released into the air.

The plague has been used as a weapon before. In 1346 the Tartar army tried to capture the port city of Caffa on the Black Sea in the Crimea. The army catapulted the bodies of plague victims over the city wall; an epidemic of plague ensued and the city surrendered.

Getting timely treatment for plague is critical. Without treatment, bubonic plague is fatal in 50 to 90 percent of cases. Septicemic plague and pneumonic plague are fatal in almost all cases if not treated within 24 to 48 hours.

Suspected plague patients are taken to a hospital and placed in isolation, where they are treated with antibiotics, intravenous * (IV) fluids, and oxygen. Anyone who has come in close contact with someone diagnosed with plague is treated with antibiotics to prevent them from contracting the infection. All suspected cases of plague must be reported to state and local health departments. Treatment and full recovery from plague can take several weeks or longer. Complications of plague include damage to vital organs due to lack of blood flow associated with sepsis, brain damage from lack of oxygen, lung damage, and death.

How Can People Prevent Becoming Infected?

Some people are at a higher risk of developing plague than others, such as lab technicians who handle the bacterium or blood samples taken from people who are infected; people who work in areas where plague occurs; and people who work with animals that carry the disease.

A person's risk of developing plague can be lowered by limiting contact with wild animals that might carry the disease; removing potential food sources and shelter for rodents near the home; treating pet dogs and cats monthly for fleas; and using insecticides to kill fleas around the home during outbreaks of plague in wild animals. Rat management in rural and urban areas also can minimize the potential for disease.

Antibiotics sometimes are prescribed to prevent infection if a person has been exposed to plague.

See also Bacterial Infections • Bioterrorism Agents: Overview • Pneumonia • Sepsis • Travel-Related Infections: Overview • Zoonoses: Overview


Books and Articles

Barford, Vanessa. “Why Hasn't the US Eradicated the Plague?” BBC News. October 15, 2015. (accessed March 12, 2016).

Jarrow, Gail. Bubonic Panic: When Plague Invaded America. Honesdale, PA: Calkins Creek, 2016.


Centers for Disease Control and Prevention. “Plague.” (accessed March 12, 2016).

National Institute of Allergy and Infectious Diseases. “Plague.” (accessed March 12, 2016).

World Health Organization. “Plague.” (accessed March 12, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed March 12, 2016).

National Institute of Allergy and Infectious Diseases. 5601 Fishers Lane, MSC 9806, Bethesda, MD 20892. Telephone: 301-496-5717. Website: (accessed March 12, 2016).

U.S. National Library of Medicine. 8600 Rockville Pike, Bethesda, MD 20894. Toll-free: 888-346-3656. Website: (accessed March 12, 2016).

World Health Organization. Avenue Appia 20, CH 1211, Geneva 27, Switzerland. Telephone: 41-22-791-21-11. Website: (accessed March 12, 2016).

* lymphatic system (lim-FAH tik) is the system that contains lymph nodes and a network of channels that carry fluid and cells of the immune system through the body.

* lymph nodes (LIMF) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

* sepsis is a potentially serious spreading of infection, usually bacterial, through the bloodstream and body.

* pneumonia (nu-MO-nyah) is inflammation of the lungs.

* meningitis (meh-nin-JY-tis) is an inflammation of the meninges, the membranes that cover and protect the brain and the spinal cord. Meningitis is most often caused by infection with a virus or a bacterium.

* abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

* clot is a thickened mass of blood cells and protein formed to stop bleeding.

* sputum (SPYOO-tum) is a substance that contains mucus and other matter coughed out from the lungs, bronchi, and trachea.

* respiratory failure is a condition in which breathing and oxygen delivery to the body are dangerously altered. This failure may result from infection, nerve or muscle damage, poisoning, or other causes.

* shock is a serious condition in which blood pressure is very low and not enough blood flows to the body's organs and tissues. Untreated, shock may result in death.

* culture (KUL-chur) is a test in which a sample of fluid or tissue from the body is placed in a dish containing material that supports the growth of certain organisms. Typically, within days the organisms will grow and can be identified.

* biopsy (BI-op-see) is a test in which a small sample of skin or other body tissue is removed and examined for signs of disease.

* intravenous (in-tra-VEE-nus), or IV, means within or through a vein. For example, medications, fluid, or other substances can be given through a needle or soft tube inserted through the skin's surface directly into a vein.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)