... countriesand finally in Brazil [1,31-33]. Variola minor accountedfor the majority of cases in the early 20th century in the United States, where it remained the only form of small-pox from the 1930s ... criterion in determining the virulenceof variola virus, the outcome of these laboratory studiesmay have been distorted by the vaccination history ofcases and maybe also by other factors. Epidemiologicalclarification ... carefully validated in earlier mathemat-ical modelling studies [16,17]. Accordingly, the policyimplications of these models differed widely, and thus the necessity arose to capture the basic...