A study has been made of the comparative virulence of several strains of vaccine virus for a number of hosts, and wide variation in animal susceptibility has been demonstrated. The results obtained in experiments with a chick-embryo-adapted strain are interpreted as indicating that the particles of virus are of essentially uniform virulence. Results of statistical analyses are presented which indicate that as the virulence of a strain of virus increases the number of elementary bodies per infectious unit approaches 1, and at that limit the chance of infection is governed primarily by the presence or absence of virus in the inoculum. With lower virulence the chance of a lesion following inoculation of virus is still described by the binomial theorem, but the actual distribution is primarily of susceptible cells not of viral particles. It is postulated that with regard to the proportion of cells available for parasitism, differences exist between different animals of a species, and that this distribution is of a normal character.

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