Definition of rickettsia
: any of a various gram-negative, parasitic bacteria (order Rickettsiales and especially genus Rickettsia) that are transmitted by biting arthropods (such as lice or ticks) and cause a number of serious diseases (such as Rocky Mountain spotted fever and typhus)
Note: Rickettsia are often pleomorphic, and may be coccoid, rod-shaped, or irregularly-shaped.
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