The Black Death, which killed an estimated 50 million people across Europe in the 14th century, never actually disappeared. The same bacterium, Yersinia pestis, still circulates among wild rodent populations on every continent except Australia and causes human infections every year. The difference today is that plague is treatable with antibiotics and kills far fewer people, but it remains a serious disease that requires urgent medical care.
Where Plague Still Occurs
The three most endemic countries are the Democratic Republic of the Congo, Madagascar, and Peru. Since the 1990s, the majority of human cases have occurred in Africa. Madagascar reports bubonic plague cases nearly every year during its epidemic season, which runs from September through April.
In the United States, an average of seven human plague cases are reported each year, ranging from zero to 17 in any given year. Almost all occur in the rural West, concentrated in two clusters: northern New Mexico, northern Arizona, and southern Colorado in one region, and California, southern Oregon, and far western Nevada in the other.
How People Get Infected Today
Plague cycles naturally among wild rodents. Rock squirrels, prairie dogs, wood rats, chipmunks, mice, and rabbits all carry the bacterium. When infected rodents die in large numbers, their fleas lose their usual hosts and go looking for new ones. That’s when people and pets are most at risk.
The most common route of infection is a flea bite, which typically causes bubonic or septicemic plague. Hunters and others who handle wild animals can also get infected through direct contact with blood or tissue, especially without gloves. The third route, and the most dangerous from a public health standpoint, is inhaling droplets coughed up by a person or animal with pneumonic plague. This is the only way plague spreads person to person, and it hasn’t been documented in the U.S. since 1924.
Cats deserve special mention. They’re highly susceptible to plague, often catching it by eating infected rodents, and can transmit infectious droplets to their owners or veterinarians. Dogs and cats that roam freely in endemic areas can also carry plague-infected fleas into the home.
The Three Forms of Modern Plague
Plague presents in three distinct ways, each with different symptoms and severity levels.
Bubonic plague is the most common form and the one most associated with the medieval pandemic. It causes fever, headache, chills, weakness, and one or more painfully swollen lymph nodes called buboes. These swellings typically appear in the groin, armpit, or neck, depending on where the flea bite occurred.
Septicemic plague occurs when the bacteria enter the bloodstream directly or when bubonic plague goes untreated and spreads. Symptoms include fever, chills, extreme weakness, abdominal pain, and shock. Bleeding can occur under the skin and in organs, and tissue on the fingers, toes, and nose can turn black and die. This blackening of tissue is what gave the medieval pandemic its name.
Pneumonic plague is the most dangerous form. It develops when bacteria infect the lungs, either from inhaling infectious droplets or as a complication of untreated bubonic or septicemic plague. Patients experience rapidly worsening pneumonia with shortness of breath, chest pain, coughing, and sometimes bloody mucus. Without treatment, pneumonic plague is nearly always fatal.
How It’s Treated
Antibiotics are effective against plague, but timing is critical. Treatment needs to begin as soon as plague is suspected, not after lab confirmation comes back. In the U.S., doctors typically use certain intravenous antibiotics as first-line therapy, continuing for 10 to 14 days or until two days after the fever breaks. When caught early, most patients recover fully.
Untreated plague is a different story. Bubonic plague kills roughly half of untreated patients, and pneumonic plague is almost universally fatal without antibiotics. The gap between treated and untreated outcomes makes rapid diagnosis one of the most important factors in survival.
One concern is antibiotic resistance. In 1995, researchers in Madagascar isolated a strain of plague that was resistant to eight different antibiotics, including all the drugs traditionally used for treatment and prevention. The resistance genes sat on a piece of DNA that could transfer to other plague bacteria. The patient survived using an alternative antibiotic, and the strain remained susceptible to several other drug classes. Only isolated cases of resistance have been documented, but the finding demonstrated that multidrug-resistant plague is biologically possible.
How Doctors Confirm a Diagnosis
The gold standard for confirming plague is growing the bacterium from a patient sample in a laboratory culture or identifying it under a microscope. Blood tests that measure the body’s immune response to the plague bacterium can also confirm a diagnosis, though this requires comparing two blood samples taken weeks apart.
For faster results, rapid diagnostic tests can detect a specific protein on the surface of the bacterium in samples from swollen lymph nodes, urine, or sputum. These tests work similarly to a home pregnancy test, producing visible lines on a strip, and are especially valuable in remote areas where full laboratory equipment isn’t available. DNA-based testing is also used, identifying plague genetic material directly from patient samples.
Reducing Your Risk in Endemic Areas
If you live in or travel to areas where plague occurs, practical prevention focuses on limiting contact with rodents and their fleas. Clear brush, rock piles, junk, and cluttered firewood from around your home, since these create rodent habitat. Store pet food and wild animal food where rodents can’t reach it, and make buildings rodent-proof by sealing gaps.
When camping, hiking, or working outdoors in endemic regions, use insect repellent containing DEET on skin and clothing, or permethrin on clothing alone. If you hunt or handle wild animals, wear gloves to avoid direct contact with blood and tissue. Keep pets on flea prevention products year-round, and don’t let dogs or cats that roam freely in endemic areas sleep on your bed. If a pet becomes visibly sick in a plague-endemic region, get veterinary care promptly.