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.