Dictionary of Human Evolution and Biology

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Very small gram-negative, bacteria-like parasites that cause disease when introduced into the cells that line small blood vessels; like viruses (and unlike true bacteria), rickettsiae require a host cell for replication; transmitted to humans by vectors such as fleas, ticks or louse bites, and a significant cause of disease (e.g. typhus) and the opportunity for natural selection. Three genera have been recognized: Rickettsia, Coxiella, and Rochalimaea.

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