Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci, bacilli, or threads. The genus was named after Howard Taylor Ricketts in honor of his pioneering work on tick-borne spotted fever.

Howard Taylor Ricketts was an American pathologist after whom the family Rickettsiaceae and the order Rickettsiales are named.