The Disease That Killed Half of Europe Still Shows Up in America’s Forests—Here’s Why

Here’s what you’ll learn when you read this story:

The “Black Death” is typically thought of as long-eradicated disease, but the plague-causing bacterium Yersinia pestis is still alive and well.

El Dorado County, which encompasses part of Lake Tahoe in California, recorded a rare case of the plague this week after someone was likely bitten by an infected flea.

While plague can now be treated thanks to antibiotics, it remains a deadly threat if detected too late or left untreated.

Over the course of human history, the three-micrometer-long Yersinia pestis—also known as the “Black Death” or “Black Plague”—has claimed an estimated 200 million human lives, making it the most acute infectious disease during our species’ short time on this planet.

Although especially devastating in Europe during the mid-1300s, where it killed 50 million people alone, Y. pestis has been with us since at least the neolithic period (later mutations are what eventually made the bacterium super virulent and deadly).

Primarily spread by infected rodents and fleas, the “Black Death” released its deadly grip on humanity’s throat as cities developed better sanitation practices and science discovered the microbial world (along with antibiotics). However, the plague was never actually eradicated, and every once in a while, an echo can be heard from one of the darkest chapters in human history.

This week, officials from El Dorado County—located just east of Sacramento, California—confirmed that someone visiting South Lake Tahoe became infected with the plague, likely being bitten by an infected flea.

“Plague is naturally present in many parts of California, including higher elevation areas of El Dorado County,” Kyle Fliflet, the county’s acting director of public health, said in a press statement. “It’s important that individuals take precautions for themselves and their pets when outdoors, especially while walking, hiking, and/or camping in areas where wild rodents are present.”

Luckily, the person identified the infection, and is now being treated by medical professionals while recovering at their home.

It’s rare to find a human case of plague in the U.S. nowadays (though it remains of particular concern in parts of Africa). The disease first entered the area in 1900, having most likely hitched a ride from one of a few overseas location known to be host to the microbe at the time.

Today, fewer than 1,000 cases are confirmed in the U.S. per year, and those can luckily be treated with antibiotics. Symptoms of plague take roughly two weeks to appear, and can include fever, nausea, weakness, and swollen lymph nodes (also known as buboes). Plague can also take three forms, and its name depends on the area of infection. The most common is bubonic plague (lymph nodes), but pneumonic (lungs) and septicemic (blood) plague can be equally deadly.

For centuries, North America was kept safe from bouts of the plague due to its geographic isolation from the Old World. Even if it managed to board a ship, long and slow voyages essentially caused infectious diseases to burn themselves out before reaching port, according to a 2024 study. But the invention and popularization of the steamship at the turn of the 20th century drastically reduced the travel times of transoceanic voyages, and suddenly made global transmission of plague a real threat.

Without wasting much time, Y. pestis arrived on the western coast of the U.S. in March 1900. Upon coming ashore, it infected upward of 280 people (and killed more than 60 percent of them), and the disease eventually found its way into native rodent populations in the western U.S.

A 2021 study suggests that the western’ U.S.’s arid soil biochemistry provides a perfect environment for the bacteria, which is why most cases are found in New Mexico and Arizona as per the Centers for Disease Control and Prevention (CDC). Conversely, this is also why the eastern U.S. remains relatively insulated from threats of plague.

While human plague cases are rare today, populations of rodents in the West (such as ground squirrels and chipmunks) are common vectors for the disease, and the South Tahoe Basin in particular is a relative hotbed for plague. A 2015 study shows that the disease flourishes in higher elevations, but not higher than roughly 6,500 feet, likely due to habitat availability (deer mice, for example, prefer trees found at this elevation). Lake Tahoe rests at around 6,200 feet in elevation.

Even so, this is the first human case reported since 2020, and only the third since 2015, when two other people were infected with plague while visiting Yosemite National Park. According to El Dorado County officials, 41 rodents have been found infected with plague from 2021 to 2024. To date, four rodents have tested positive for the deadly diseases in 2025.

Your best bet for protection against this medieval scourge is to avoid sick, dead, or injured rodents and wear insect repellent containing DEET to ward off any plague-carrying fleas. Pets are also susceptible to plague, so it’s also best to keep animals on leash when venturing into plague-prone areas.

Thankfully, the plague—bubonic or otherwise—isn’t the scourge it once was. But it still remains a lingering threat from our medieval past.

Photo credit: Hearst Owned