How does the Oriental rat flea spread bubonic plague?

The Oriental rat flea (Xenopsylla cheopis) is considered the most efficient carrier of the bubonic plague bacterium, Yersinia pestis.12

The transmission process occurs as follows:

• Ingestion and Blockage: When a flea feeds on an infected rodent, it ingests the plague bacilli. These bacteria multiply within the flea's digestive tract until they create a blockage.3
• Regurgitation: Because the digestive tract is obstructed, the flea becomes unable to properly digest its next meal. When it bites a new host—such as a human—it regurgitates the infected material into the bite wound.3
• Infection: Once inside the human host, the bacteria migrate through the lymphatic system to the lymph nodes, where they colonize and cause the characteristic swelling known as a bubo.3

This cycle typically intensifies when an epizootic outbreak kills off the local rodent population, forcing hungry, infected fleas to seek new hosts among humans.13