Rickettsia are a group of bacteria that cause a number of serious human diseases, including the spotted fevers and typhus. Rod- or sphere-shaped, rickettsia lack both flagella (whip-like organs that allow bacteria to move) and pili (short, flagella-like projections that help bacteria adhere to host cells). Specific species of rickettsia include Rickettsia rickettsii, which causes the dangerous Rocky Mountain spotted fever; R. akari, which causes the relatively mild rickettsial pox; R. prowazekii, which causes the serious disease epidemic typhus; R. typhi, the cause of the more benign endemic or rat typhus; and R. tsutsugamushi, the cause of scrub typhus.

Rickettsia are transmitted to humans by insects such as ticks, mites, and chiggers. Usually the insect has acquired the bacteria from larger animals which they parasitize, such as rats, mice, and even humans. When an insect infected with rickettsia bites a human, the...