Disease Information: Plague
When you hear about “the plague,” you probably think of the Middle Ages, when the Bubonic form of plague killed millions of people throughout Europe, Asia and Africa. In Europe, as many as 2.5 million people—one third of the population—died between 1347 and 1350. It has been said that there weren’t enough people left alive to bury the victims. At the time, people didn’t know what caused plague and no cure was available. So there was panic whenever an outbreak occurred. Many works of literature and art depict the terror surrounding the plague and the horrible effects it had on the population.
Maybe you think plague is gone and part of history, like the Middle Ages. If so, you’ll be surprised to learn that it is alive and well. Today, we have protective vaccines and medications that cure people with plague, so it takes far fewer lives than other deadly diseases. But the name alone continues to cause fear. And although we know much more about plague than medieval people did, it continues to kill people even today
Plague is a disease that affects animals and humans. It is caused by the bacterium, Yersinia pestis. This bacterium was named after Alexandre Yersin, the scientist credited with discovering how the disease spread during an epidemic in 1894. (How would you like to have a deadly bacterium named after you?)
Yersinia pestis bacteria are carried by fleas and the wild rodents on which they live (often rats and squirrels). Plague outbreaks are rare these days, but still can happen in places where infected rodents and their fleas live in people’s homes. In the Middle Ages, it was much more common for homes to be infested with rats and fleas—which is one reason why so many people were infected with plague then.
There are three different kinds of plague. Bubonic plague is an infection of the lymph nodes, which are glands located throughout the body that help to fight off illness by acting as filters for bacteria and viruses. Septicemic plague is an infection of the blood. Pneumonic plague is an infection of the lungs. The type of plague a person gets depends on how he or she was infected in the first place. Septicemic plague can cause a victim’s skin to turn very dark purple. That’s why plague sometimes was called the “Black Death.”
The most recent outbreak of plague in the U.S. was in 1924. But it still exists here, mostly in the Southwest and Midwest. And while only about 2,000 cases of plague are reported worldwide each year, it remains a very serious disease. If you get plague and don’t get treatment, it can kill you.
One Small Pest = A Huge Health Risk
Most often, plague is spread when an infected flea bites a person, or when someone handles an animal infected with plague. It also is possible to catch plague through the air, if someone with pneumonic plague sneezes near you.
A few days after infection, sudden fever, chills, headache, nausea, weakness, and painful, swollen lymph nodes may develop. These are symptoms of plague. The disease advances quickly, so it is important to see a doctor as soon as possible after infection. Most plague patients who are treated quickly and properly with antibiotics will recover fully. But if left untreated, plague can invade the lungs and bloodstream. Once in the lungs, plague can be spread by sneezing or coughing, and it is far more difficult to cure. About half of all people with this kind of plague die.
A Disease that Won't Die
Plague is not likely to be eradicated (eliminated). Even with new technology, improved conditions, and good healthcare in most modern cities, plague and Yersinia pestis bacteria remain strong opponents. In fact, overcrowding, combined with a lack of proper sanitation and pest control in some poorer countries, has increased the chances for another plague outbreak. Like the fleas that carry it, this disease is tough to kill.
Keywords: Yersinia pestis | bacteria | disease | epidemic | microbe | microbiology | microorganism | pandemic | pathogen | plague | vaccine
- Photo of Yersinia pestis by Larry Stauffer, Oregon State Public Health Laboratory, and the CDC/1918.
- Moreno, N., Tharp, B., Erdmann, D, Rahmati Clayton, S., and Denk, J. (2012) The Science of Microbes Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-54-5
- Photo of flea courtesy of the CDC/2025.
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