Plague is an infectious disease caused by the bacterium Yersinia pestis, one of the most virulent microorganisms known to science. It primarily circulates among rodents and their fleas, but it can spread to humans through flea bites, contact with infected animals, or inhaling contaminated respiratory droplets. Though often associated with medieval history, plague still infects people today, with most modern cases occurring in Africa, parts of Asia, and the rural western United States.
The Bacterium Behind Plague
Yersinia pestis is a small, rod-shaped bacterium that belongs to the same family as common gut bacteria. Despite that seemingly ordinary classification, it is extraordinarily dangerous. Fewer than 20 individual bacteria introduced through the skin can cause a lethal infection. The bacterium achieves this partly by hijacking immune cells called macrophages, essentially hiding inside the very cells meant to destroy it and using them as a safe space to multiply early in infection.
Y. pestis evolved from a much milder soil-dwelling relative less than 10,000 years ago. In the wild, it maintains itself in a cycle between rodents and fleas. The bacterium can survive for years in the dark, cool environment of rodent burrows, meaning new animals that move into an abandoned burrow can become infected long after the previous occupants died.
Three Forms of the Disease
Plague takes three distinct clinical forms depending on where the bacteria concentrate in the body. Each progresses differently and carries different risks.
Bubonic Plague
This is the most common and most recognizable form. It develops after an infected flea bites a person, and symptoms appear within 2 to 8 days. The hallmark sign is one or more swollen, painful lymph nodes called buboes, typically in the groin, armpit, or neck, near the site of the flea bite. Fever, headache, chills, and weakness accompany the swelling. Without treatment, the bacteria can spread from the lymph nodes into the bloodstream or lungs, triggering the more dangerous forms of the disease. Untreated bubonic plague kills about 66% of those infected. With antibiotics, that drops to roughly 13%.
Septicemic Plague
Septicemic plague occurs when the bacteria enter or spread into the bloodstream. It can develop on its own from a flea bite or animal contact, or it can follow untreated bubonic plague. Symptoms include fever, chills, extreme weakness, abdominal pain, and shock. One of the most striking features is tissue death: skin, fingers, toes, and the nose can turn black as blood flow is disrupted. Because these symptoms overlap with other serious infections, septicemic plague is frequently diagnosed late, which makes it more deadly than the bubonic form.
Pneumonic Plague
Pneumonic plague is the most serious form. It develops when bacteria reach the lungs, either by spreading from another part of the body or by being inhaled directly from an infected person or animal’s cough. Symptoms come on fast, sometimes within a single day of exposure: fever, headache, weakness, and a rapidly worsening pneumonia with shortness of breath, chest pain, and coughing that may produce bloody mucus. This is the only form of plague that spreads person to person, through close contact with respiratory droplets. Untreated pneumonic plague is almost always fatal.
How Plague Spreads
The primary route of transmission is the bite of an infected flea. Fleas pick up the bacteria by feeding on infected rodents, including rock squirrels, prairie dogs, chipmunks, wood rats, mice, and voles. When a flea then bites a human, the bacteria enter through the skin.
Handling a dead or sick animal is another route. Hunters, trappers, or anyone skinning wild rodents in areas where plague exists can become infected through breaks in the skin. Cats deserve special mention here: they are particularly susceptible to plague and often catch it by hunting infected rodents. A cat with pneumonic plague can cough infectious droplets directly onto its owner.
Person-to-person transmission only happens with pneumonic plague and requires close, direct contact with someone who is coughing. Bubonic and septicemic plague do not spread between people.
Plague in the Modern World
Plague is rare but not gone. The United States reports an average of seven human cases per year, mostly in northern New Mexico, northern Arizona, southern Colorado, and parts of California, Oregon, and Nevada. Globally, most cases since the 1990s have occurred in Africa. Nearly all modern infections happen in small towns, villages, and agricultural areas rather than cities.
The disease is treatable with several classes of antibiotics, and early treatment dramatically improves survival. The challenge is that plague’s early symptoms, particularly fever, chills, and weakness, look like many other infections. What distinguishes bubonic plague is the rapid appearance of extremely painful, swollen lymph nodes. Septicemic and pneumonic plague are harder to identify early, which is why they remain more dangerous even with modern medicine.
The Three Great Pandemics
Plague has shaped human history more than almost any other disease. The first recorded pandemic, the Justinian Plague, began in 541 CE and killed nearly 100 million people across Asia, Africa, and Europe over the following years. The second pandemic, known as the Black Death, struck in 1347 and killed at least 25 million people in Europe and another 25 million in Asia and Africa, wiping out a quarter of Europe’s population in just three years. The third pandemic began in Yunnan, China, in 1894 and spread globally over the next several decades, causing over 15 million deaths (mostly in India) before subsiding in 1959.
Before antibiotics existed, fatality rates for all forms of plague ranged from 66% to 93%. The introduction of effective antibiotics in the mid-20th century transformed plague from a near-certain death sentence into a survivable illness, provided treatment begins early enough.
Reducing Your Risk in Endemic Areas
If you live in, work in, or visit areas where plague circulates in wildlife, practical steps can significantly lower your risk. Reduce rodent habitat around your property by clearing brush, rock piles, junk, and cluttered firewood. Remove outdoor food sources that attract rodents, including unsecured pet food and wild animal feed. Make buildings rodent-proof by sealing gaps and openings.
When camping, hiking, or working outdoors, use insect repellent containing DEET on skin and clothing, or permethrin on clothing alone. If you have pets, keep them on flea control products year-round in endemic areas, and don’t let free-roaming dogs or cats sleep on your bed. Wear gloves when handling or skinning any wild animals. If your pet becomes unusually sick in an area where plague is known to occur, get veterinary care quickly, both for the animal’s sake and to protect everyone in the household.