The median initial R across the 30 detailed outbreaks was 2 with a range of 0 to 38 (Table 3 , Figure 2a ). About a third had an initial R of 1 or less, and about two thirds had an initial R of 3 or less. Outbreaks that remained in the community had a lower initial R (median 1) than those that were hospital based from the first generation (median 12) ( Table 3 , Figure 3a ). Mixed outbreaks, which generally started in the community and later spread through hospital contacts, had an intermediate initial R (median 2) ( Table 3 , Figure 3a ). The initial R was smaller when the index case(s) had a typical versus atypical or hemorrhagic presentation (medians 1 vs. 4 or 5, respectively), and when the index case was identified as smallpox versus unrecognized (median 1 vs. 5) ( Table 3 , Figures 3b and  3c) . The effect was most pronounced in hospital outbreaks -median initial R's for identified versus unidentified index cases in hospital outbreaks were 3 versus 15, but the same values for community and mixed outbreaks were 1 versus 4, and 2 versus 2, respectively.