The Third Pandemic

The Third Pandemic, which began in Yunnan province in China during the 1850s, was the third great outbreak of plague in recorded history, following the Plague of Justinian in the sixth century and the Black Death in the fourteenth. The pandemic spread through global trade networks to kill millions across Asia, India, and beyond, and it led to the identification of Yersinia pestis as the cause of plague — the scientific breakthrough that made the modern understanding of the Black Death possible.

Origins in China

The Third Pandemic emerged in the mountainous region of Yunnan in southwestern China, where Yersinia pestis circulated among wild rodent populations. The disease spread from the rural hinterland to the cities of the province, and from there along the trade routes that connected Yunnan to the rest of China and to Southeast Asia.

By the 1890s, the pandemic had reached the southern Chinese cities of Canton and Hong Kong. The outbreak in Hong Kong in 1894 attracted international attention and prompted the scientific investigation that would identify the causative organism.

The Discovery of Yersinia pestis

In June 1894, two bacteriologists arrived independently in Hong Kong to investigate the plague outbreak: the Swiss-French physician Alexandre Yersin, working for the Pasteur Institute, and the Japanese physician Kitasato Shibasaburō. Both isolated a rod-shaped bacterium from the lymph nodes of plague victims, but it was Yersin who correctly identified the organism and demonstrated its pathogenicity in animal experiments.

The bacterium was initially named Pasteurella pestis and later reclassified as Yersinia pestis in Yersin’s honour. The discovery established plague as a bacterial disease and opened the way for the development of diagnostic tests, vaccines, and, eventually, antibiotics.

In the following years, the transmission cycle of plague was elucidated. The French physician Paul-Louis Simond demonstrated in 1898 that fleas transmitted the disease from rats to humans, and the British entomologist A. W. Bacot described the “blocked flea” mechanism in 1914.

The Global Spread

From Hong Kong and the ports of southern China, the Third Pandemic spread through maritime trade networks to every inhabited continent. The disease reached India in 1896, where it killed millions over the following decades. It spread to Southeast Asia, Japan, Australia, South Africa, South America, and the western United States.

The impact of the Third Pandemic was most severe in India, where the British colonial administration’s response to the plague — including forced quarantine, house searches, and the segregation of the sick — generated widespread resentment and contributed to the growth of the Indian independence movement. The plague killed approximately twelve million people in India between 1896 and 1930.

In the United States, plague was introduced through the port of San Francisco in 1900, likely carried by ships from Asia. The disease established itself among the wild rodent populations of the western states, where it persists to the present day as an enzootic infection.

The Impact on Plague Science

The Third Pandemic was the context in which the scientific understanding of plague was developed. The identification of Yersinia pestis, the elucidation of the rat-flea transmission cycle, and the development of the first effective treatments all occurred during this pandemic. The scientific knowledge gained during the Third Pandemic provided the framework for understanding the earlier pandemics, including the Black Death.

The DNA studies that have confirmed the identity of the medieval plague strain have also revealed that the Black Death strain was the ancestor of nearly all modern plague-causing bacteria. This finding establishes a direct genetic link between the Third Pandemic and the Black Death, demonstrating that the bacterium responsible for the fourteenth-century catastrophe is still circulating in the world today.