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June 1894Peer-reviewed · 2 sourcesWell documented

Alexandre Yersin Identifies the Plague Bacterium During the Third Pandemic

A Pasteur Institute scientist isolates the bacterium behind the Black Death during a Hong Kong outbreak, closing a question that had stood for over five centuries

On the timeline · around June 1894 · The Birth of Vaccination and EpidemiologyThe Birth of Vaccination and EpidemiologyModern Pandemics and Modern MedicineAlexandre Yersin Identifies the Plague Bacterium During the Third Pandemic18251850187519001925

Quick facts

Discoverer
Alexandre Yersin, Pasteur Institute
Location
Hong Kong
Date
June 1894
Pandemic context
Third Plague Pandemic, spread globally via steamship rat populations
Bacterium named
Yersinia pestis, in Yersin's honor

What happened

A bubonic plague outbreak began in Hong Kong in early May 1894, part of what became the Third Plague Pandemic as infected rats spread the disease along steamship routes from southern China to port cities worldwide over the following decades. In June 1894, Alexandre Yersin, a Swiss-born French bacteriologist with the Pasteur Institute, traveled to Hong Kong and isolated a gram-negative bacillus from the buboes of plague victims, publishing his findings in the Annales de l'Institut Pasteur. His experiments met Koch's postulates for proving causation, and his description of the bacterium's staining properties proved more accurate than a competing claim published around the same time by the Japanese bacteriologist Kitasato Shibasaburo. The organism was later named Yersinia pestis in Yersin's honor. Researchers in the following years established that the bacterium was transmitted between rats and humans by fleas, completing the transmission picture.

Why it matters

This closed the question of what pathogen had caused the Plague of Justinian and the Black Death, both later confirmed by ancient DNA to be the same species Yersin isolated in 1894, connecting three pandemics spanning thirteen centuries to a single identified organism. Confirming rat fleas as the vector also gave public health authorities a concrete, actionable target: rat control became a standard plague-prevention measure worldwide.

How we know

Yersin's original 1894 publication in the Annales de l'Institut Pasteur documents his isolation method; subsequent transmission studies on rats and fleas, and much later ancient DNA sequencing of medieval and 6th-century remains, independently confirmed that the bacterium he isolated matched the pathogen behind the earlier pandemics.

Sources

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Part of a timelinePandemics Through History24 events · How plague, pox, and pandemic flu remade societies, and how the science of germs and vaccines fought backView all →