Plague is a serious bacterial infection caused by a germ called Yersinia pestis, spread mainly through the bites of infected fleas. It’s the same disease behind the Black Death that killed tens of millions of people in the 14th century, and it still exists today, with cases occurring in parts of Africa, Asia, and the western United States. Left untreated, some forms are nearly 100% fatal, but modern antibiotics have made plague survivable when caught early.
How Plague Spreads
Plague bacteria live in a cycle between wild rodents and the fleas that feed on them. When an infected flea bites a human, the bacteria enter the body and begin multiplying. Many types of wild animals carry the disease, including rock squirrels, wood rats, ground squirrels, prairie dogs, chipmunks, mice, and rabbits.
Flea bites are the most common route of transmission, but they’re not the only one. Handling an infected animal without gloves can introduce bacteria through breaks in the skin. Domestic cats are a particular concern: cats that hunt rodents can develop plague and transmit infectious droplets to their owners through coughing or close contact. The pneumonic form of plague can also spread directly from person to person through respiratory droplets, making it the only form that doesn’t require an animal or flea intermediary.
The Three Forms of Plague
Bubonic Plague
This is the most common and most recognizable form. After a flea bite, the bacteria travel to the nearest lymph node and multiply there, producing a painful, swollen lump called a bubo. Symptoms appear within 2 to 8 days and include sudden fever, headache, chills, and weakness alongside one or more of these swollen nodes, typically in the groin, armpit, or neck. Without treatment, bubonic plague can progress into either of the two more dangerous forms.
Septicemic Plague
When the bacteria enter the bloodstream, they cause septicemic plague. This can happen as the initial presentation or as a complication of untreated bubonic plague. Symptoms include fever, chills, extreme weakness, abdominal pain, and shock. One of the most distinctive signs is tissue death in the extremities: skin on the fingers, toes, and nose can turn black as blood flow is cut off. Internal bleeding into the skin and organs is also possible.
Pneumonic Plague
The rarest and most dangerous form develops when the bacteria infect the lungs. It causes a rapidly progressing pneumonia with shortness of breath, chest pain, coughing, and sometimes bloody or watery mucus. Pneumonic plague is the only form that spreads person to person through coughs and is almost universally fatal without treatment. Even with antibiotics, the death rate is around 17%, based on a meta-analysis of cases from 1946 to 2017. Without any treatment, that figure rises to 98%.
Where Plague Still Exists
Plague has never been eradicated. It persists in stable pockets across the Americas, Africa, and parts of Asia. Most human cases since the 1990s have occurred in Africa, where periodic outbreaks still happen. Madagascar, the Democratic Republic of the Congo, and Peru are among the countries that report cases regularly.
In the United States, plague occurs in the rural West. The two main hotspots are northern New Mexico, northern Arizona, and southern Colorado in one cluster, and California, southern Oregon, and far western Nevada in the other. Cases are uncommon, typically numbering in the single digits per year, but they do happen, usually among people who live or recreate in areas where wild rodents carry the disease.
Three Pandemics That Shaped History
Plague has caused three major pandemics. The first, the Justinian Plague of 541 CE, swept through Asia, Africa, and Europe, killing nearly 100 million people between 542 and 546. The second, the Black Death, arrived in Europe in 1347 and killed a quarter of the continent’s population, over 25 million people, with another 25 million dying in Asia and Africa during the same period. The third pandemic began in Yunnan, China, in 1894, spread to Hong Kong and India, and continued for decades until 1959, causing over 15 million deaths, most of them in India.
The third pandemic was also when scientists finally identified the bacterium responsible. That discovery, along with the development of antibiotics in the 20th century, transformed plague from a civilization-ending catastrophe into a treatable infection.
Diagnosis and Treatment
Doctors diagnose plague by drawing blood cultures or taking a sample from a swollen lymph node. The bacteria can be identified under a microscope using special stains, or grown in a lab culture, which remains one of the most reliable methods. If cultures come back negative but plague is still suspected, blood tests for antibodies can confirm the diagnosis.
Speed matters enormously. Treatment with antibiotics lasts 10 to 14 days and should begin as soon as plague is suspected, without waiting for lab confirmation. Pneumonic plague in particular can become fatal within days of the first symptoms. The specific antibiotic depends on the form and severity, but several effective options exist, including both injectable and oral medications. When treatment starts early, outcomes improve dramatically, especially for bubonic plague, which has a much lower fatality rate than the pneumonic form.
Reducing Your Risk in Endemic Areas
If you live, work, or camp in parts of the western U.S. where plague is present, a few practical steps lower your risk significantly. Remove brush, rock piles, junk, and cluttered firewood around your home to reduce rodent habitat. Store pet food and wild animal feed where rodents can’t reach it, and make buildings as rodent-proof as possible.
When hiking or camping, use insect repellent containing DEET on skin and clothing. Products with permethrin can be applied to clothing for additional flea protection. If you hunt or handle wild animals, wear gloves to avoid direct skin contact with potentially infected tissue.
Pets deserve attention too. Apply flea control products regularly, especially to dogs and cats that spend time outdoors. Free-roaming animals are more likely to encounter infected rodents or pick up infected fleas and bring them inside. Cats are especially vulnerable to plague and can pass it on through close contact, so keeping them off your bed in endemic areas is a reasonable precaution.