Variola virus strains of 1960--1975: the range of intraspecies variability and relationship between properties and geographic origin.

A comparative study of 61 variola virus strains recovered from patients in various geographic regions of the world revealed significant intraspecies variability in some biological properties, established the range of this variability and demonstrated the existence of strains with atypical properties. There was a certain correlation between some laboratory markers of variola virus and the degree of its pathogenicity for man. The prevalence of more virulent and thermoresistant strains in the Hindustan subcontinent and the peculiarity and heterogeneity of the strains circulating in Africa were demonstrated. Alastrim virus was detected among the African isolates.