title: Severity of smallpox and virulence of causative virus strains.
mesh_terms: D000818:Animals; D002642:Chick Embryo; D003588:Cytopathogenic Effect, Viral; D007194:India; D051379:Mice; D012901:Variola virus / Q000472:pathogenicity*; D014774:Virulence; D014776:Virus Cultivation