n 1  cap  :  the type genus of the family Rickettsiaceae comprising rod-shaped, coccoid, or diplococcus-shaped often pleomorphic bacteria that live intracellularly in biting arthropods (as lice or ticks) and when transmitted to humans by the bite of an arthropod host cause a number of serious diseases (as Rocky Mountain spotted fever and typhus)  2  pl  -sias  or  -si*ae  also  -sia  :  any bacterium of the order Rickettsiales and esp. of the family Rickettsiaceae .
 
 
 
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