The disease is most common in the southwestern and Gulf states. The disease is commonly spread from rat to rat, and from rat to human by the oriental and northern rat fleas. It has also been transmitted by cat fleas from infected feral cats. Fleas are also vectors of tularemia, a disease related to plague. The natural reservoirs of tularemia are cottontail rabbits in the East, and jack rabbits in the West.
Most cases reported are by hunters.
Fleas can also be intermediate hosts of several species of tapeworm including species which parasitize humans, dogs, and cats. The simplest is to count and collect fleas landing or crawling on an observer's lower legs for one minute. Light-colored trousers are preferred to provide greater contrast and facilitate counting and collection. Fleas may also be combed off animals for an index of animal infestation. It is dark in color.