The Third Plague was a massive pandemic of bubonic plague that began in Yunnan, China, in 1855 and lasted over a century, not officially ending until 1959. It killed an estimated 12 to 25 million people worldwide, with India bearing the worst of it by far. It was also the pandemic that finally revealed what causes plague: a bacterium now called Yersinia pestis, spread to humans through the bites of infected fleas.
The name “Third Plague” places it in a sequence. The First Plague Pandemic was the Plague of Justinian in the 6th century. The Second was the Black Death of the 14th century and its recurring waves across Europe. The Third Plague Pandemic was the modern chapter, and it reshaped both global public health and our scientific understanding of infectious disease.
How It Started in Yunnan
The pandemic’s origins trace to southwestern China, where a convergence of environmental disruption set the stage. Widespread deforestation driven by mining and agriculture pushed wild rodents out of their natural habitats and into closer contact with common rats living near human settlements. This allowed the plague bacterium to jump from wildlife into the rat populations that lived alongside people, and from there into humans via flea bites.
For decades, the disease circulated in Yunnan and neighboring regions. It reached the major port cities of Guangzhou (Canton) and Hong Kong by 1894, and that was the turning point. Once plague hit busy international shipping hubs, infected rats and their fleas traveled aboard cargo ships to port cities across the globe.
Where It Spread
By the turn of the 20th century, outbound ships from Hong Kong had seeded outbreaks in Bombay, Calcutta, Cape Town, and San Francisco. The disease reached nearly every inhabited continent. India was hit hardest. Over 75 percent of all recorded deaths from the pandemic occurred there. In 1906 alone, India recorded more than a million plague deaths, with nearly half concentrated in the Punjab region.
In the United States, the first confirmed case appeared in San Francisco on March 6, 1900. Federal health officials had anticipated it, knowing the city received heavy ship traffic from Asian ports with active epidemics. By mid-May, with 11 confirmed cases in San Francisco’s Chinatown and many more suspected, authorities declared an epidemic. The response was complicated by political resistance. California’s governor and business interests refused to publicly acknowledge the outbreak, fearing economic damage. The standoff eventually required mediation in President McKinley’s office, and federal health authorities were allowed to take over plague control only on the condition that state officials could save face by never admitting plague had been present.
The Discovery That Changed Medicine
Before 1894, nobody knew what caused plague. The Hong Kong outbreak that year drew two bacteriologists racing to find the answer. Shibasaburo Kitasato, trained under the legendary Robert Koch, arrived in Hong Kong on June 12 with a large team and a well-equipped laboratory. Alexandre Yersin, a researcher from the Pasteur Institute in Paris, arrived three days later with far fewer resources.
Both men worked independently to isolate the organism responsible. Yersin succeeded in identifying the bacterium clearly and accurately. The pathogen was eventually named Yersinia pestis in his honor. This discovery was a landmark in microbiology, turning plague from a mysterious curse into a disease with a known cause and a traceable transmission chain.
How Plague Spreads
The bacterium maintains itself in a cycle between rodents and their fleas. When a flea feeds on an infected rodent, it picks up the bacteria. When that rodent dies (as many do during outbreaks), the flea seeks a new host. If that new host is a person, the flea’s bite delivers the bacteria into the bloodstream.
This is why the Third Plague Pandemic followed trade routes so effectively. Rats lived on ships, fleas lived on rats, and when ships docked in new ports, the cycle found fresh populations of rodents and people. The disease could establish itself in a new city within weeks of a single infected ship arriving.
The Death Toll
Estimates of total mortality range from 12 to 25 million people over the pandemic’s full duration. The wide range reflects the difficulty of counting deaths across decades and dozens of countries with varying record-keeping. India accounted for the overwhelming majority. Half of all global deaths, by some estimates, occurred there. China, where the pandemic began, also suffered heavily in its early decades, though precise figures from 19th-century Yunnan are harder to pin down.
The pandemic did not end with a single decisive moment. Cases gradually declined through the early and mid-20th century as public health measures improved, rat control programs expanded, and antibiotics became available. The World Health Organization considered the pandemic to have officially ended in 1959, more than a hundred years after it began.
Plague After the Pandemic
The Third Plague Pandemic left a lasting mark. Yersinia pestis established itself in wild rodent populations on continents where it had never existed before, including North America. In the western United States, the bacterium now circulates permanently among rock squirrels, prairie dogs, wood rats, chipmunks, and other wildlife. Most human cases in the U.S. occur in two clusters: one spanning northern New Mexico, northern Arizona, and southern Colorado, and another across California, southern Oregon, and far western Nevada.
Globally, most human plague cases since the 1990s have occurred in Africa. The disease is treatable with antibiotics when caught early, but it still kills when diagnosis is delayed. The Third Plague Pandemic may be over, but the bacterium it spread around the world remains a permanent part of the landscape.