Why can plague persist in wild rodent populations?

Plague persists in wild rodent populations primarily because the causative bacterium, Yersinia pestis, maintains an enzootic cycle within these animal communities.1

The survival of the pathogen depends on the relative resistance of the host. While some rodents, such as the black rat (Rattus rattus) and the Norway rat (R. norvegicus), are highly susceptible to the disease—meaning the infection often kills the host and thus fails to persist—the plague bacillus survives by circulating among more resistant wild rodent species. In these populations, the infection can remain present at all times, affecting only small numbers of animals without causing widespread mortality that would eliminate the host population.21

This cycle is facilitated by fleas, which act as vectors. When the bacteria are ingested by a flea, they multiply and block the insect's digestive tract, ensuring that the pathogen is transmitted to new hosts during subsequent bites. Because it is impossible to eliminate these wild rodent reservoirs, the disease remains a permanent feature in certain regions of the world.134