Which bacterium causes Rocky Mountain spotted fever, a tick-borne disease?

Rickettsia rickettsii causes Rocky Mountain spotted fever, a tick-borne disease.

R. rickettsii is an obligate intracellular bacterium, meaning it must live and multiply inside host cells. It is transmitted mainly by infected hard ticks, including the Rocky Mountain wood tick, American dog tick, and brown dog tick. The illness can produce fever, headache, and a characteristic rash, although the rash may appear late or be absent.

The disease was recognized in the early twentieth century, and Howard Taylor Ricketts helped establish its infectious cause. The genus Rickettsia was named in his honor. Rocky Mountain spotted fever occurs in several parts of the United States, not only in the Rocky Mountains.

R. rickettsii is often confused with Borrelia burgdorferi, the cause of Lyme disease. Both are tick-borne, but they belong to different bacterial groups and cause distinct illnesses. Prompt medical evaluation matters because untreated Rocky Mountain spotted fever can become severe.