The illness is caused by an intracellular parasite called Rickettsia rickettsii , which is released by the tick while it is feeding. This parasite is found most often in the Dermacentor andersoni in western North America and in Dermacentor variabilis in the east. There are between about 600 and 1200 cases a year in the USA alone, with most cases occurring between April and September. People with dogs are also at increased risk. The illness has an incubation period of 5-7 days after being bitten by an infected tick.
Treatment consists of antibiotics, most often at least initially in the hospital setting.
Treatment continues until two days after all the symptoms have resolved. Although there is usually a good prognosis when treatment is started soon enough, people do die from RMSF if treatment is delayed. These carriers tend to remain in remote areas. The incidence of this disease is unknown, due mainly to the frequency with which the diagnosis is missed. Peak incidence of cases is mid-summer.