Understanding Plague

by National Institute of Health

Plague is an infectious disease caused by bacteria called Yersinia pestis. These bacteria are found mainly in rodents, particularly rats, and in the fleas that feed on them. Other animals and humans usually contract the bacteria from rodent or flea bites.

Historically, plague destroyed entire civilizations. In the 1300s, the "Black Death," as it was called, killed approximately one-third (20 to 30 million) of Europe's population. In the mid-1800s, it killed 12 million people in China. Today, thanks to better living conditions, antibiotics, and improved sanitation, current World Health Organization statistics show there were 2,118 cases in 2003 worldwide.

Transmission

Yersinia pestis is found in animals throughout the world, most commonly in rats but occasionally in other wild animals, such as prairie dogs. Most cases of human plague are caused by bites of infected animals or the infected fleas that feed on them. In almost all cases, only the pneumonic form of plague (see below) can be passed from person to person.

Forms of Plague

Y. pestis can affect people in three different ways: bubonic, septicemic, or pneumonic plague.

Bubonic plague

In bubonic plague, the most common form, bacteria infect the lymph system and become inflamed. (The lymph or lymphatic system is a major component of your body's immune system. The organs within the lymphatic system are the tonsils, adenoids, spleen, and thymus.)

How is it contracted? Usually, bubonic plague is contracted by being bitten by an infected flea or rodent. In rare cases, Y. pestis bacteria, from a piece of contaminated clothing or other material used by a person with plague, enter through an opening in your skin.

What are the symptoms? Bubonic plague affects the lymph nodes (another part of the lymph system). Within three to seven days of exposure to the bacteria, flu-like symptoms will develop such as fever, headache, chills, weakness, and swollen, tender lymph glands (called buboes-hence the name bubonic).

Is it contagious? Bubonic plague is rarely spread from person to person.

Septicemic plague

This form of plague occurs when the bacteria multiply in the blood.

How is it contracted? Septicemic plague is contracted the same way as bubonic plague-usually through a flea or rodent bite. Septicemic plague also can appear as a complication of untreated bubonic or pneumonic plague.

What are the symptoms? Symptoms include fever, chills, weakness, abdominal pain, shock, and bleeding underneath the skin or other organs. Buboes, however, do not develop.

Is it contagious? Septicemic plague is rarely spread from person to person.

Pneumonic plague

This is the most serious form of plague and occurs when Y. pestis bacteria infect the lungs and cause pneumonia.

How is it contracted? Pneumonic plague can be contracted in one of two ways.

Primary pneumonic plague is contracted when plague is inhaled. This type of plague can be spread to someone else.

Secondary pneumonic plague develops when bubonic or septicemic plague goes untreated after the disease has spread to the lungs. At this point, the disease can be transmitted to someone else.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.