Fig 1. Transmission cycle of Y. pestis in a plague-endemic community [37].

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.