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a tribe of bacteria of the family Rickettsiaceae, order Rickettsiales, made up of small, pleomorphic, mostly intracellular organisms, which are classified in three genera, Coxiella, Rickettsia, and Rochalimaea. They occur as parasites in arthropods and cause disease in vertebrate hosts. Pathogenic species fall generally into four groups, based upon immunologic characterization: I, classic typhus (also causing epidemic typhus, brill-zinsser disease, and murine typhus); II, spotted fever (causing rocky mountain spotted fever, boutonneuse fever, rickettsialpox, Siberian tick typhus, and Queensland tick typhus); III, scrub typhus; and IV, miscellaneous organisms (including the causes of q fever and trench fever). Called also Rickettsiae.