Forty-nine post-1945 European and North American outbreaks were identified. Thirty-six of these were included in a single World Health Organization (WHO) publication [21] and prior work by T.M. Mack [12] . Several outbreaks were documented in multiple publications (Tables 1 and  2 ). Four outbreaks were excluded. The source of the first outbreak could not be confirmed as the outbreak may have resulted from the importation of smallpox on a rug (Union of Soviet Socialist Republics or U.S.S.R, 1959). The second and third outbreaks occurred under unique circumstances; one occurred aboard a ship (Poland, 1962) and the other may have been the result of testing aerosolized smallpox (U.S.S.R, 1972) [21] [22] [23] . A fourth outbreak was excluded because the number of cases in each generation was not documented; therefore, the initial reproductive rate (R) could not be derived from the data (United States or U.S., 1946) [24] . The remaining 45 original outbreaks occurred over a thirty-year period in different countries and under different circumstances. The majority of reported outbreaks occurred in three coun- (Tables 1 and 2 ). Most resulted from involved index cases imported from endemic countries, but two outbreaks were the result of laboratory exposure [25, 26] .