Rickettsia /riket′sē·ə/ [Howard T. Ricketts, American pathologist, 1871–1910] , a genus of microorganisms that combines aspects of both bacteria and viruses. They can be observed with a light microscope, divide by fission, and may be controlled with antibiotics. They also exist as viruslike intracellular parasites, living in the intestinal tracts of insects such as lice. Thus a human infested with lice is also likely to be infected with a form of typhus transmitted by Rickettsia prowazeki. Rickettsial diseases have been responsible for many of history’s worst epidemics. The various species are distinguished on the basis of similarities in the diseases they cause. The spotted fever group includes diseases such as Rocky Mountain spotted fever and rickettsialpox; the typhus group includes epidemic typhus and murine typhus; and a miscellaneous group includes Q fever and trench fever. Rickettsial diseases are uncommon in parts of the world where insect and rodent populations are well controlled. −rickettsial, adj.
