Fig 1. Transmission cycle of Y. pestis in a plague-endemic community.

This figure was originally published by Ben Ari et al. [37] which was made available under the Creative Commons CC0 public domain dedication. Under favourable environmental conditions, populations of rodent species that are very susceptible to plague infection (T. leucogaster and Mastomys coucha) increase to high levels [38]. If these population increases occur in an area where there is a quiescent plague focus, the plague may break out in the susceptible rodent population. In plague-endemic areas, this population increase is crucial in plague transmission because a large number of mice and rats correspond to a large number of fleas [38]. Plague kills the susceptible rodents, and their infected fleas leave the carcass and seek new hosts, thereby spreading the infection rapidly throughout areas of high population.