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Why was the Oriental rat flea an especially important plague carrier?

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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.

The Oriental rat flea (Xenopsylla cheopis) is considered the most efficient carrier of plague because of its specific biological interaction with the plague bacillus (Yersinia pestis).1

When the flea ingests the bacteria from an infected host, the bacilli multiply within the flea’s digestive tract until they create a blockage. This obstruction causes the flea to regurgitate the bacteria into a new host—whether a rodent or a human—during subsequent attempts to feed. Because these fleas readily infest humans when their primary rodent hosts die off during an epizootic outbreak, they serve as a critical bridge for transmitting the disease to human populations.21

While more than 100 species of fleas can be infected by the plague bacillus, the Oriental rat flea’s behavior and its role in the rodent-flea-human cycle make it uniquely significant in the spread of the disease.1