The shorthand “rats spread plague” hides a more complex ecology. Yersinia pestis circulates through networks of mammals and fleas, and the important species differ by region.
Reservoirs, susceptible hosts and vectors
Some mammal populations can maintain the bacterium at low levels. Other susceptible species may experience conspicuous die-offs when transmission rises. Fleas connect hosts and can sometimes bite people after an infected animal dies.
In western North America, ground squirrels, prairie dogs, wood rats, chipmunks, deer mice and voles can be part of local plague ecology. In Central Asian foci, marmots have important historical and present-day relevance. Domestic cats and dogs do not create the natural focus, but they can encounter infected animals or fleas and bring exposure closer to households.
Why animal detections are not human cases
Finding Y. pestis in a flea, rodent or other animal is evidence of ecological activity. It should trigger appropriate veterinary and public-health attention, but it cannot be added to a human case count. The unit, case definition and meaning are different.
Reading ecological signals
Animal surveillance, flea testing and reports of die-offs can help investigators identify where transmission may be elevated. Their absence is not proof that the bacterium is absent, because surveillance coverage is uneven.
Plague Map therefore treats natural foci and animal detections as contextual layers. They explain why human spillover is possible without implying that an outbreak is underway.