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How does Yersinia pestis spread from rodents to humans?

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two plague victims with sores lying in bed, while a robed figure gestures above them. — plague Image depicting victims of the sixth biblical plague.

Yersinia pestis primarily spreads from rodents to humans through the bite of infected fleas.123

This transmission cycle typically occurs when an epizootic outbreak kills off a population of rodents. Once the original rodent host dies, the fleas—which carry the bacteria in their digestive tracts—seek a new host and begin to infest humans. When these fleas bite a human, they regurgitate the Yersinia pestis bacilli into the new host, leading to infection.4

While flea bites are the primary mode of transmission, humans can also become infected through other forms of contact, including:

Direct contact with the bodily fluids or tissues of an infected animal.15
Scratches or bites from infected animals, such as domestic cats.15

Once inside the human body, the bacteria typically migrate through the lymphatic system to the lymph nodes, where they multiply and cause the characteristic swellings known as buboes.45