Caused by R. rickettsii, RMSF has the highest case-fatality rate of all rickettsial diseases.

• In the United States, the prevalence is highest in the south-central and southeastern states. Most cases occur between May and September.
• A rare presentation of fulminant RMSF is seen most often in male black pts with G6PD deficiency.
• RMSF is transmitted by different ticks in different geographic areas—e.g., the American dog tick (Dermacentor variabilis) transmits RMSF in the eastern two-thirds of the United States and in California, and the Rocky Mountain wood tick (D. andersoni) transmits RMSF in the western United States.