rickettsiae

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rickettsiae (ri-ket-si-ee) pl. n. (sing. rickettsia) a group of very small nonmotile spherical or rodlike parasitic bacteria that cannot reproduce outside the bodies of their hosts. They cause such illnesses as rickettsial pox, Rocky Mountain spotted fever, and typhus. Rickettsiae infect arthropods (ticks, mites, etc.), through which they can be transmitted to humans.
rickettsial (ri-ket-si-ăl) adj.