Tiny microorganisms that live in cells and are considered somewhere between bacteria and viruses in structure.
A group of very small nonmotile spherical or rodlike parasitic organisms. They resemble bacteria in their cellular structure and method of asexual reproduction but, like viruses, they cannot reproduce outside the bodies of their hosts. Rickettsia infect arthropods (ticks, mites, etc.), through whom they can be transmitted to mammals (including man), in which they can cause severe illness. The species Rickettsia akari causes rickettsial pox; R. conorii, R. mooseri. R. prowazekii, R. and quintana R. tsutsugamushi cause different forms of typhus, and R. rickettsii causes Rocky Mountain spotted fever.