Why do flea bites transmit plague bacteria?

Fleas transmit the plague bacteria, Yersinia pestis, because of a mechanical blockage in their digestive systems. Once a flea ingests the bacteria from an infected host, the bacilli multiply within the flea’s digestive tract until it becomes completely blocked.1

When the hungry, infected flea subsequently bites a new host—such as a human or another rodent—it is unable to properly ingest blood due to this blockage. Consequently, the flea regurgitates the plague bacilli directly into the wound of the new host. Once inside the host, the bacteria migrate through the lymphatic system to the lymph nodes, where they produce proteins that weaken the host's immune response, allowing the infection to colonize and spread.1