Plague is one germ with several faces. The same bacterium can swell a gland, flood the blood, or settle in the lungs, and where it lands decides how sick a person gets and whether they can pass it on. It still turns up today, including in the rural American West, and ordinary antibiotics cure it when they are started early.
Most people meet plague in history class. In the 1300s the Black Death swept across Europe, and the World Health Organization (WHO) puts the toll there at more than 50 million deaths. The germ behind it, a bacterium called Yersinia pestis, never went away. It lives on in wild rodents and their fleas on every continent except Oceania, and it still spills over into people every year.
What surprises many readers is that “plague” is not one illness. Doctors split it into types by where the bacteria do their damage: the lymph nodes, the bloodstream, or the lungs. Those types spread differently, look different, and carry very different risks for the people nearby.
One germ, three main forms
Think of the body as a house with three places an intruder can hide. Lymph nodes are the bean-sized filters of the immune system, clustered in the groin, armpits, and neck. The bloodstream is the plumbing that reaches everything. The lungs are the one room with a window to the outside, which matters because whatever is in them can be coughed out.
The U.S. Centers for Disease Control and Prevention (CDC) and WHO both name three main forms of plague, matching those three places: bubonic, septicemic, and pneumonic. A case can start as one form and turn into another. Untreated bubonic plague can move into the blood, and plague in the blood can seed the lungs.
How plague reaches people
Plague is a zoonosis, a disease that normally lives in animals and only sometimes jumps to humans. In nature it cycles between wild rodents and the fleas that feed on them. According to the CDC, people catch it in three main ways.
- Flea bites. This is the most common route. When plague kills a lot of rodents in one area, their hungry fleas look for a new meal, and a person or pet can be next. Dogs and cats can carry infected fleas into the house. A bite usually leads to bubonic or septicemic plague.
- Handling an infected animal. Touching the blood or tissue of a sick or dead animal can pass the bacteria through small breaks in the skin. The CDC’s example is a hunter skinning a rabbit without gloves.
- Breathing in droplets. A person or animal with plague in the lungs can cough out tiny droplets loaded with bacteria. Breathing those in, usually during close contact, causes pneumonic plague. Cats are especially vulnerable to plague, and in the United States a sick cat is a known source of these cases.
Bubonic plague: the swollen-gland form
Bubonic plague is the most common type worldwide, and the CDC says it makes up more than 80% of cases in the United States. After a flea bite, the bacteria travel to the nearest lymph node and multiply there. The node becomes swollen, tense, and very painful. That lump is called a bubo, the old word that gave bubonic plague its name. It usually shows up in the groin, armpit, or neck, close to where the bite happened.
Symptoms usually start 2 to 8 days after infection, a stretch doctors call the incubation period. They include sudden fever, chills, headache, and weakness, along with one or more of those painful swollen nodes. WHO also lists body aches, nausea, and vomiting.
Bubonic plague rarely spreads from person to person. The danger is what happens if it is not treated. The bacteria can escape the lymph node into the blood or the lungs, and WHO estimates that 30% to 60% of untreated bubonic cases are fatal.
Septicemic plague: when it gets into the blood
Septicemia is a medical word for an infection that has spread through the bloodstream. In septicemic plague the bacteria multiply in the blood itself. It can be the first form a person gets, from a flea bite or from handling an infected animal, or it can develop when bubonic plague goes untreated.
The CDC lists fever, chills, extreme weakness, belly pain, shock (a dangerous drop in blood pressure), and bleeding into the skin and organs. Skin and tissue can turn black and die, especially on the fingers, toes, and nose. That tissue death is called necrosis.
Septicemic plague can be hard to spot early because there may be no obvious swollen node. The CDC notes it can start with stomach symptoms such as nausea, vomiting, diarrhea, and belly pain. WHO says it is fatal if left untreated.
Pneumonic plague: the lung form, and the one that spreads between people
Pneumonic plague is plague in the lungs, a fast and severe pneumonia. It comes in two versions, and the difference is simply how the bacteria got there.
- Primary pneumonic plague starts in the lungs. A person breathes in droplets coughed out by someone, or some animal, that already has pneumonic plague.
- Secondary pneumonic plague starts somewhere else. Bacteria from untreated bubonic or septicemic plague travel through the body and reach the lungs.
Either way, the CDC describes fever, headache, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery mucus. After breathing in the bacteria, symptoms usually start within 1 to 3 days, and sometimes in about a day.
This is the form public health officials worry about most, for two reasons. First, it is the only form of plague that can spread from person to person, through droplets coughed out during close contact. Once one person with secondary pneumonic plague starts coughing, the people around them can catch primary pneumonic plague, and WHO warns that this chain can fuel severe epidemics. Second, it moves fast. WHO says untreated pneumonic plague can kill within 18 to 24 hours of the first symptoms, while recovery rates are high when treatment starts within about a day.
That is why health departments move quickly around a pneumonic case. Patients are isolated, and the people who had close contact with them are tracked down and offered preventive antibiotics, a step called post-exposure prophylaxis. The CDC notes that person-to-person spread has not been documented in the United States since 1924, but rare pneumonic cases still occur in people exposed to sick cats.
Rarer forms of plague
Doctors also see a few unusual forms. The CDC lists pharyngeal, meningeal, and cutaneous (skin) plague as rare presentations.
- Pharyngeal plague is plague of the throat. According to CDC treatment guidelines published in 2021, it follows contamination of the mouth and throat with infected material, such as undercooked meat from an infected animal. Typical signs are an inflamed sore throat and swollen glands in the neck.
- Meningeal plague, or plague meningitis, is an infection of the thin layers that wrap the brain and spinal cord. It usually appears as a complication when another form of plague was treated late or not well enough, and it brings fever, a stiff neck, and confusion. In one series of 105 American patients described in the same CDC guidelines, 6% developed it.
Treatment today: antibiotics, and fast
The good news is that plague is a bacterial infection, and common antibiotics work against it. WHO and the CDC both stress that early diagnosis and treatment save lives. In the United States, the CDC lists the antibiotics gentamicin and the fluoroquinolone family as first-line choices, given by IV or by mouth depending on how sick the patient is.
Speed matters more than almost anything else. The CDC tells doctors to start treatment as soon as plague is suspected, without waiting for lab results, because the severe forms can turn deadly within a day or two. There is no routine plague vaccine for the public. WHO recommends vaccination only for high-risk groups, such as some lab and health workers.
Where plague still occurs
Plague is rare, but it is not gone. WHO says animal plague is found on every continent except Oceania. Since the 1990s, most human cases have been reported in Africa, and during 2019 to 2025 most cases reported to WHO came from the Democratic Republic of the Congo and Madagascar. In Madagascar, bubonic cases show up nearly every year during the season that runs from September to April.
In the United States, plague arrived in 1900 on rat-infested steamships, and the first outbreaks hit port cities. The last urban epidemic was in Los Angeles in 1924 and 1925. After that, the bacteria moved from city rats into wild rodents such as ground squirrels and prairie dogs, and they have stayed in the rural West ever since.
Today the CDC counts an average of about seven human cases a year in the United States, with a range of zero to 17. Most are picked up in two regions: northern New Mexico, northern Arizona, and southern Colorado, and a second area covering California, southern Oregon, and far western Nevada. Cases in the eastern states have been in people who traveled from the West or who were exposed in a lab.
If you live in or visit those areas, the CDC and WHO advice is mostly common sense:
- Do not touch sick or dead rodents, rabbits, or other wild animals, and wear gloves if you have to handle one.
- Keep pets on flea control, and do not let dogs and cats roam where rodents live.
- Guard against flea bites outdoors, and cut back brush, woodpiles, and food sources that draw rodents near the house.
- Take a sick pet, especially a sick cat, to a vet promptly, and mention if it may have caught a rodent.
Why the Black Death still matters
Plague has been with people for a very long time. A 2020 review in Clinical Microbiology Reviews notes that ancient DNA shows the bacterium infecting human populations in Eurasia for at least 5,000 years. The Black Death of the 1300s was its most famous outbreak, and art from the centuries that followed still carries the shock of it.
The difference now is knowledge. Medieval doctors had no idea that fleas and rats were involved, or that one cough could carry the disease from one person to the next. Today a doctor who suspects plague can start antibiotics the same day, and health officials know to look for the fleas, the dead animals, and the close contacts.
The bottom line
Plague is one bacterium with three main faces. Bubonic plague settles in a lymph node and causes a painful swollen bubo. Septicemic plague floods the blood and can turn skin black. Pneumonic plague fills the lungs, and it is the one that can spread from person to person through coughing. Rarer forms can strike the throat or the lining of the brain.
All of them are treatable with antibiotics, and all of them are far more dangerous when treatment is late. Plague is rare, even in the American West. But if you get sudden fever and a painful swollen gland, or a fast-worsening cough, after contact with fleas, rodents, or a sick pet, treat it as an emergency and see a doctor right away.