Although Numerous sources now consider other factors or dismiss the idea that the Black Death was carried by rats, the CDC continues this claim. That issue is not not my interest, but a historical perspective is. \

Were late 19th century scientists, with their advancing medicinal view of the world, aware that fleas on rats carried the disease?

Current views of that time state that the Catholic Church considered cats the cause of the plague, and persecuted them, so it can be assumed that it wasn't entirely common knowledge that rats spread the disease. Through searching I cannot find an answer. When was when this discovery Made: that fleas on rats were carriers of Bubonic Plague?

  • 1
    How do you persecute a cat? Jan 30, 2017 at 13:06
  • 3
    Spray it with cold water? Undermine its position as the internet's number one obsession?
    – Ne Mo
    Jan 30, 2017 at 13:52
  • 3
    That link does not (as the question text claims) say that cats were blamed for the plague. If anything, it actually itself claims the opposite (lack of cats caused the plague). Considered opinion of the time was that diseases like plague traveled through bad air ("malaria"), and based on that Pope Clement was advised to keep a fire around him at all times.
    – T.E.D.
    Jan 30, 2017 at 14:25
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    Since no one knows for sure that Black Death was "discovery" is maybe not the best word.
    – Greg
    Feb 1, 2017 at 10:39

1 Answer 1


I am not entirely sure of what you're question is but maybe this answer will clarify from a French Wiki page:

Researchers working in Asia during the "Third Pandemic" identified plague vectors and the plague bacillus. In 1894, in Hong Kong, Swiss-born French bacteriologist Alexandre Yersin isolated the responsible bacterium (Yersinia pestis) and determined the common mode of transmission. Japanese physician and researcher Kitasato Shibasaburō initially misidentified the bacterium. In 1898, French researcher Paul-Louis Simond demonstrated the role of fleas as a vector.

The disease is caused by a bacterium usually transmitted by the bite of fleasfrom an infected host, often a black rat. The bacteria are transferred from the blood of infected rats to the rat flea (Xenopsylla cheopsis). The bacillus multiplies in the stomach of the flea, blocking it. When the flea next bites a mammal, the consumed blood is regurgitated along with the bacillus into the bloodstream of the bitten animal. Any serious outbreak of plague in humans is preceded by an outbreak in the rodent population.

During the outbreak, infected fleas that have lost their normal rodent hosts seek other sources of blood. The bacterium that causes this disease, Yersinia pestis, was named after Yersin. His discoveries led in time to modern treatment methods, including insecticides, the use of antibiotics and eventually plague vaccines.

The British colonial government in India pressed medical researcher Waldemar Haffkine to develop a plague vaccine. After three months of persistent work with a limited staff, a form for human trials was ready. On January 10, 1897 Haffkine tested it on himself. After the initial test was reported to the authorities, volunteers at the Byculla jail were used in a control test, all inoculated prisoners survived the epidemics, while seven inmates of the control group died. By the turn of the century, the number of inoculees in India alone reached four million. Haffkine was appointed the Director of the Plague Laboratory (now called Haffkine Institute) in Bombay.

Third plague pandemic

  • Please add a link, make your quote a blockquote, and please add paragraph breaks.
    – Spencer
    Jan 29, 2017 at 16:52
  • Sorry - To Clarify, I was asking when scientists discovered that the plague was transmitted by fleas on rats (Although some people might disagree, but that doesn't concern me) - Which, as you answered - was 1894! Thank you very much. Jan 29, 2017 at 20:06
  • But as others have already stated there was no reports of mass death of rats during the Black Death.
    – liftarn
    Jan 30, 2017 at 11:32

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