title: Concerning the question of infraforms in Rickettsia prowazeki.
mesh_terms: D000818:Animals; D002642:Chick Embryo; D005374:Filtration; D008722:Methods; D008851:Micropore Filters; D012283:Rickettsia prowazekii / Q000166:cytology* / Q000254:growth & development / Q000302:isolation & purification; D014461:Ultracentrifugation