Yersinia pestis is transmitted by the bite of a rat flea.

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The main concept here is vector-borne transmission by fleas. Yersinia pestis is carried by fleas, especially rat fleas, which become infected after feeding on rodents. The bacteria multiply in the flea’s gut and can block the proventriculus, causing the flea to regurgitate infectious material into the bite wound when it feeds on a new host. This flea-driven transmission is the classic route for plague to humans. Other listed routes don’t fit this primary mode—the waterborne route isn’t how plague is spread, and while pneumonic plague can spread via respiratory droplets between people, that’s a different transmission scenario from the flea bite.