The rickettsioses are spread by hard (ixodid) ticks or mites, and generally occur worldwide. The causative agent of RMSF is Rickettsia rickettsii. In northwestern US and western Canada, it is spread by the Rocky Mountain wood tick - Dermacentor andersoni. In eastern North America, the American dog tick, D. variabilis, is responsible.
The tick may inject the rickettsia directly, after being attached for several hours, or its feces or crushed body may infect the bite wound or open sores.