When you hear "Black Death," your brain probably goes straight to 14th-century Europe—think plague doctors in those creepy bird masks and carts full of bodies. It feels like ancient history. But if you’re hiking through the Sierra Nevadas or strolling around a campground in Lake Tahoe, the black plague in california is a very modern reality.
It’s weird to think about.
We live in this high-tech hub of the world, yet a disease that wiped out a third of Europe is literally living in the dirt and the squirrels right outside our back door. It's not a "gone" thing. In fact, California is one of the few places in the United States where Yersinia pestis—the bacteria behind the plague—is essentially a permanent resident.
How the Plague Hit San Francisco and Stayed
The story of the black plague in california didn't start in the Middle Ages. It started in 1900. A ship called the S.S. Australia sailed into San Francisco Bay, and hidden in its cargo were rats carrying infected fleas. On March 6, 1900, a city health officer found the body of Wong Chut King in a hotel basement in Chinatown. He had the telltale buboes—swollen, painful lymph nodes—of the bubonic plague.
The response was a total mess.
The Governor at the time, Henry Gage, actually tried to cover it up because he didn't want to hurt the state's economy. He literally denied the plague existed even as people were dying. He called it "plague fake" and attacked the federal health officers who were trying to warn the public. It was a disaster of politics over science. By the time the state finally admitted there was a problem, the bacteria had jumped from city rats to rural squirrels.
That was the turning point. Once the plague moved into the wild rodent population, it became "endemic." That means it’s part of the ecosystem now. You can’t exactly vaccinate every ground squirrel in the Mojave Desert or the San Bernardino Mountains.
Where the Bacteria Hides Today
You aren't going to catch the plague from another person in a Starbucks. That’s not how this works anymore. In California, the plague cycle is almost entirely about fleas and rodents.
Ground squirrels are the big ones. Chipmunks, too. Even woodrats. These little guys carry the fleas that carry the bacteria. Most of the time, it stays in the "sylvatic" (wild) cycle, meaning it just circulates among the animals. But every now and then, a "die-off" happens. If you see a bunch of dead squirrels in one area, that’s a massive red flag. When the host dies, the fleas get hungry and start looking for the next warm body.
Sometimes that body belongs to a hiker. Sometimes it’s a house cat.
South Lake Tahoe is a frequent hotspot. In 2020, a resident there tested positive for the plague—the first human case in California in five years. Before that, in 2015, two visitors caught it after hanging out at Yosemite National Park. One was at the Crane Flat Campground, which ended up getting shut down so authorities could treat the squirrel burrows with insecticide. It’s a constant cat-and-mouse game between the California Department of Public Health (CDPH) and a microscopic killer that’s been around for millennia.
It's not just the mountains
While the Sierra Nevada range and the Foothills are the usual suspects, the black plague in california shows up in unexpected spots. It’s been found in rodents in Los Angeles County, Kern County, and even down in San Diego. The bacteria prefers cooler, more humid environments, which is why the mountains are prime real estate, but it’s surprisingly hardy.
What it Actually Feels Like (The Biology)
If you get bitten by an infected flea, you usually have about two to six days before things get ugly. It starts out like a really bad flu. Fever. Chills. Headaches. But then the "bubo" appears. This is a lymph node—usually in the groin, armpit, or neck—that swells up to the size of a golf ball. It’s incredibly painful. This is the bubonic form.
If the bacteria gets into your lungs, it becomes pneumonic plague. This is the scary version because it can spread through the air from person to person. However, we haven't seen a person-to-person transmission of plague in California since the 1920s.
Then there’s septicemic plague, where the bacteria multiplies in the bloodstream. This is where the "black" in Black Death comes from. The skin and tissues start to die and turn black, especially on the fingers, toes, and nose.
The good news? We have antibiotics. Streptomycin, gentamicin, or doxycycline usually clear it right up if you catch it early. The bad news? If you don't treat it, the mortality rate is north of 50%.
Why We Don't Have a Massive Outbreak
You might wonder why, if the plague is just sitting there in the woods, we aren't all dying.
💡 You might also like: Getting the Tdap Vaccine in Pregnancy: Why Timing is Everything for Your Baby
It's mostly because humans don't interact with fleas that much anymore. Better housing, better sanitation, and the fact that we don't sleep on piles of straw helps a lot. Also, the CDPH Vector-Borne Disease Section is actually really good at their job. They constantly trap rodents and test fleas for the presence of Y. pestis. When they find a "hot" area, they post signs, close campgrounds, and use "flea dust" (insecticide) in the burrows to kill the carriers.
But nature is unpredictable.
Climate change is starting to shift where these rodents live. Warmer winters might mean more fleas survive the season. Drier summers might push rodents closer to human campsites in search of food or water. The boundaries are blurring.
Protecting Your Pets (The Weak Link)
The biggest risk factor for most Californians isn't actually hiking; it’s their pets.
Cats are extremely susceptible to the plague. They hunt the squirrels, get bitten by the fleas, or eat the infected meat. A sick cat can then bring those fleas into your bed. Or, if the cat develops the pneumonic version, it can cough the bacteria directly into your face. Dogs are a bit more resistant—they usually just get a fever and a swollen node—but they can still bring "hitchhiker" fleas into the house.
If you live in a rural or mountainous area, your flea control game needs to be 100% on point.
Real-World Precautions That Actually Matter
Forget the masks. You don't need them. What you do need is a basic understanding of how to share space with California wildlife without bringing a medieval disease home.
First, stop feeding the squirrels. I know they’re cute. I know they’ll eat a Cheeto right out of your hand at a lookout point. Don't do it. When you encourage squirrels to gather in high-traffic human areas, you're creating a plague factory.
Second, check the signs. If a trailhead has a warning about plague activity, take it seriously. It means they’ve found the bacteria in the last few weeks.
Third, wear DEET. It's not just for mosquitoes. It keeps the fleas off your ankles. If you're walking through tall grass or brush in the Sierras, wear long pants and tuck them into your socks. You’ll look like a nerd, but you won't get bubonic plague. It's a fair trade.
The Reality Check
The black plague in california is a reminder that we aren't as separated from nature as we think. We’ve paved over the valleys and built mansions on the hillsides, but the microscopic world doesn't care about property lines. The plague is a permanent part of the Golden State's landscape.
It isn't a reason to panic or stop going outside. It's just a reason to be smart. We live in an era where a disease that once collapsed empires can be cured with a week of pills. But you have to know it's there to treat it.
Actionable Steps for Your Next Trip
- Check the CDPH website for recent plague activity reports before heading to major parks like Yosemite or Tahoe.
- Keep your distance from any wild rodent that seems sluggish or "tame"—that’s a classic sign of illness.
- Apply veterinary-approved flea treatment to your dogs and cats at least 48 hours before entering mountain or desert regions.
- Avoid "stealth camping" in areas with visible rodent burrows or heavy populations of ground squirrels.
- Seek medical attention immediately if you develop a sudden high fever and painful swelling within a week of being in the California backcountry. Tell the doctor exactly where you were; many city doctors won't even think to test for plague unless you prompt them.
The plague isn't a ghost of the past. It’s a neighbor. Treat it with the respect an apex predator deserves, even if that predator is just a few microns wide.