The word plague evokes images of medieval Europe, the bubonic horror that claimed millions of lives in the 14th century. Yet, despite its centuries-old origins, the plague—caused by the bacterium Yersinia pestis—has never been fully eradicated. It still exists today in wildlife reservoirs, occasionally spilling over into human populations in rare but serious cases.
What is the Plague?
The plague is an infectious disease caused by the bacterium Yersinia pestis, a microorganism that primarily lives in rodents and is spread by fleas. Known for its role in some of the deadliest pandemics in human history, the plague can infect both humans and animals.
Key Facts:
Causative Agent: Yersinia pestis, a Gram-negative bacterium.
Transmission: Most often through flea bites, but can also occur from handling infected animals or inhaling respiratory droplets from a person with pneumonic plague.
Reservoir Hosts: Wild rodents such as rats, ground squirrels, prairie dogs, and chipmunks act as natural carriers.
Forms of the Disease:
Bubonic plague – causes painful, swollen lymph nodes (buboes).
Septicemic plague – spreads into the bloodstream, leading to organ failure and tissue death.
Pneumonic plague – infects the lungs and can spread between people through cough droplets.
Why It Still Matters:
Even though modern antibiotics make plague treatable, its potential severity and historical legacy make it a disease of concern. In places like the western United States, Madagascar, and parts of Africa, plague still emerges from wildlife reservoirs, reminding us that it has not disappeared.
1. Historical Toll: The Black Death and Beyond
During the 14th century, the Black Death ravaged Europe, killing an estimated 50 million people, roughly 50% of the population at the time. Across human history, Y. pestis has claimed approximately 200 million lives globally, earning its notoriety as one of the deadliest infectious diseases.
In the United States, plague came to the West Coast around 1900, carried by steamships from Asia. Between 1900 and 1904, San Francisco Chinatown suffered the first significant bubonic plague outbreak in the continental U.S., with 121 confirmed cases and 119 deaths.
One of the most dramatic episodes occurred in 1924, when a pneumonic plague outbreak in Los Angeles claimed numerous lives. This form, infecting the lungs, was both severe and contagious but eventually contained through quarantine and public health measures.
2. Understanding the Plague Today: Symptoms, Causes, Forms, and Treatment
a) Causative Agent & Transmission
The plague is caused by the bacterium Yersinia pestis. It is primarily transmitted through the bite of an infected flea, which typically has fed on wild rodents—such as ground squirrels, chipmunks, rats, and others. Direct contact with infected animals or their tissues can also lead to transmission.
In the U.S., plague persists mainly in rural and higher-elevation areas across the western states, where these animal reservoirs exist naturally.
b) Forms & Symptoms
There are three main forms of plague:
Bubonic plague: The most common form—marked by swollen lymph nodes (buboes), fever, weakness, and headache.
Septicemic plague: Occurs when the infection spreads through the bloodstream; can cause abdominal pain, bleeding into skin, tissue death.
Pneumonic plague: Infects the lungs; symptoms include cough, shortness of breath, chest pain, and may be contagious via airborne droplets.
Symptoms typically appear within 1–7 days of exposure, often around two weeks for bubonic plague.
c) Diagnosis & Treatment
Diagnosis usually involves detecting Y. pestis in lymph node aspirate, blood, or sputum samples.
Treatment is highly effective if started early: antibiotics such as gentamicin, fluoroquinolones (e.g., ciprofloxacin or doxycycline), streptomycin, chloramphenicol, or tetracyclines can reduce fatality risk from over 70% (if untreated) to around 10% with treatment.
No widely available vaccine exists in the U.S. Vaccines once existed but have been discontinued due to safety and efficacy concerns.
d) Prevention & Public Health Measures
Efforts focus on vector control and avoiding contact with wild rodents and their fleas. Preventive steps include:
Avoiding sick, dead, or injured rodents.
Not feeding or handling wild rodents.
Preventing pets from roaming or interacting with rodents; using flea control on pets.
Using DEET-based insect repellent, wearing long clothing, avoiding burrow areas, and heeding posted warnings when outdoors.
Health agencies monitor rodent populations; surveillance is key to detecting outbreaks.
3. Plague in the Modern Era: Rare but Present
Globally, roughly 600 cases per year are reported, with the highest incidence in countries like the Democratic Republic of the Congo, Madagascar, and Peru.
In the U.S., plague remains rare, averaging 7–9 human cases annually, typically in the western states—New Mexico, Arizona, Colorado, California, Oregon, and Nevada.
From 1970 through 2020, 496 cases were reported in the U.S.—underscoring its relative scarcity.
4. Latest: New Case Reported in California (August 2025)
a) The Incident
In late August 2025, El Dorado County officials confirmed that a South Lake Tahoe resident tested positive for the plague after a camping trip, most likely contracting it via an infected flea bite.
The individual is recovering at home under medical care, supported by antibiotics.
b) Local Surveillance
Between 2021 and 2024, 41 rodents (ground squirrels or chipmunks) in El Dorado County tested positive for plague exposure. In 2025, four additional rodents tested positive— all within the Tahoe Basin.
c) Historical Context of California Cases
This is the first human case in El Dorado County since 2020, also linked to South Lake Tahoe. Before that, two human cases occurred in 2015 at Yosemite National Park. Those were the first reported human plague cases in California since 2006, with all prior individuals recovering.
According to CDC data, the U.S. sees around seven plague cases annually, mostly in western states.
d) Public Health Advisory
El Dorado County officials, including Kyle Fliflet, emphasized that plague occurs naturally in parts of California—especially at higher elevations—and urged outdoor visitors and residents to take precautions for themselves and their pets.
Key recommendations include avoiding contact with wild rodents, placing pets on leashes, using DEET insect repellent, and keeping pets from burrow areas.
e) Broader Recognition
Media outlets—including Popular Mechanics, The Guardian, Time, People, Axios, and CIDRAP—have widely reported the incident, confirming both its rarity and the effectiveness of antibiotic treatment when detected early.
5. Summary Overview Table
| Area | Key Details |
|---|---|
| Historical Impact | Black Death killed ~50 million in 14th century; first U.S. outbreak in SF (1900–1904); fatal LA outbreak in 1924 |
| Causes & Transmission | Bacterium Yersinia pestis; primarily via flea bites and contact with infected rodents |
| Forms & Symptoms | Bubonic (buboes), septicemic (blood), pneumonic (lungs, contagious) |
| Treatment | Effective antibiotics—gentamicin, doxycycline, streptomycin; untreated fatality >70%, with treatment ~10% |
| Prevention | Avoid rodents/fleas, use DEET, monitor pets, vector control, surveillance |
| Modern Cases | ~600 global/year; ~7–9 U.S. cases annually; most in western states |
| 2025 California Case | South Lake Tahoe, August; infected via flea; recovering; rodents tested positive; precautions advised |
Final Thoughts
The recent case in California is a vivid reminder that the plague, though relegated to the annals of history, remains a real—but manageable—public health threat when vigilance and prompt medical intervention are applied.
Learning from the past—and safeguarding today—is key: early detection, antibiotic access, vector control, and public awareness keep this ancient menace at bay.
