Bubonic plague is caused by the bacterium Yersinia pestis, which spreads to humans primarily through the bites of infected fleas. The disease still occurs naturally in parts of the western United States, Africa, Central Asia, and South America, where the bacterium circulates among wild rodent populations. Understanding how the bacterium moves from animals to fleas to humans explains why plague persists centuries after the infamous medieval pandemics.
How Fleas Transmit the Bacterium
The most common route of infection starts with a flea feeding on an infected rodent. Once inside the flea’s gut, Yersinia pestis does something unusual: it forms a sticky biological mass called a biofilm inside a valve in the flea’s throat. This mass grows until it partially or completely blocks the flea’s ability to swallow blood. When the flea bites its next host, incoming blood hits the bacterial blockage and gets pushed back out into the bite wound, carrying bacteria with it. The flea is essentially regurgitating bacteria every time it tries to feed.
This blockage also makes the flea hungrier, since blood can’t reach its stomach properly. A blocked flea bites more aggressively and more often, increasing the chances it will pass the infection along. The oriental rat flea is the most efficient transmitter, but other flea species found on ground squirrels and prairie dogs in the American West can also carry and spread the bacterium.
What Happens Inside the Body
After bacteria enter through the bite, there’s an incubation period of 2 to 8 days before symptoms appear. During that window, the bacteria travel from the bite site through the lymphatic system to the nearest lymph node. What makes Yersinia pestis so dangerous is its ability to hijack the body’s own immune cells. When white blood cells called macrophages engulf the bacteria (a normal immune response), the bacteria survive and multiply inside them rather than being destroyed. They use that protected environment to produce proteins that make them resistant to further immune attack.
Once the bacteria emerge from these cells, they’re essentially armored against the immune system and multiply rapidly in the lymph node. This produces the hallmark symptom: a bubo, an extremely painful, swollen lymph node that can grow to the size of an egg. Buboes typically appear in the groin, armpit, or neck, depending on where the flea bite occurred. As the infection progresses, the lymph node tissue becomes severely damaged, with internal bleeding and tissue death. Patients develop fever, headache, chills, and weakness alongside the swelling.
Without treatment, the mortality rate ranges from 66% to 93%. With antibiotics, that drops to roughly 16%, which is still significant and makes early treatment critical.
Other Ways Humans Get Infected
Flea bites aren’t the only route. Hunters, trappers, or anyone handling a dead or sick animal can become infected through direct contact with its blood or tissue. Skinning a rabbit or prairie dog without gloves, for example, can allow bacteria to enter through small cuts in the skin. This type of exposure typically causes either bubonic plague (with swollen lymph nodes) or septicemic plague (where bacteria enter the bloodstream directly).
A third form, pneumonic plague, develops when the infection reaches the lungs. A person or animal with pneumonic plague can cough bacteria-containing droplets into the air, and inhaling those droplets is the only way plague spreads directly between people. This is the rarest but most dangerous form. Person-to-person transmission hasn’t been documented in the United States since 1924, though occasional cases still occur from exposure to infected cats. Cats are particularly susceptible to plague, often catching it from rodents they hunt, and can transmit respiratory droplets to their owners or veterinarians.
The Animal Reservoirs That Keep Plague Alive
Yersinia pestis doesn’t need humans to survive. It persists in cycles among wild rodent populations, and different species serve as long-term hosts on every continent where plague is found. In North America, ground squirrels, chipmunks, prairie dogs, and wood rats are the primary carriers. In Central Asia, marmots and gerbils maintain the bacterium, with marmots capable of harboring it through their long hibernation periods. South American outbreaks involve wild guinea pigs and related species, while in southern Africa, gerbils and field mice carry the infection through the interior regions.
These animal reservoirs are why plague can’t be eradicated. The bacterium cycles through rodent populations continuously, occasionally spilling over into species that live closer to humans or into flea populations that bite people.
Where Plague Occurs Today
In the United States, most human cases cluster in two regions: northern New Mexico, northern Arizona, and southern Colorado form one hotspot, while California, southern Oregon, and far western Nevada make up the other. The bacterium arrived in American port cities during the early 1900s, then spread from urban rats to rural rodent species and became permanently established in the western landscape.
Globally, plague remains active in parts of sub-Saharan Africa (particularly Madagascar and the Democratic Republic of Congo), Central Asia, and South America. Cases tend to be rural and tied to areas where people live or work near wild rodent habitats.
Climate Conditions That Trigger Outbreaks
Plague outbreaks are seasonal, and temperature plays a major role. The bacterium spreads most effectively when temperatures fall between roughly 12°C and 22°C (about 53°F to 71°F), with peak transmission around 17°C (63°F). In African countries, plague incidence has been associated with a slightly broader range of 15°C to 27°C. Moderate humidity, between 60% and 80%, also favors outbreaks. Historical data from India spanning five decades showed that plague was more likely to emerge under these moderate humidity conditions compared to very dry or very wet years.
These conditions aren’t random. They reflect the sweet spot where flea populations thrive and rodent activity is high. Extremely hot or cold temperatures suppress flea reproduction, while moderate warmth and humidity create ideal conditions for flea survival and the rodent population booms that bring more infected animals into contact with each other and with human settlements.