The recent literature contains several estimates for the reproductive rate of smallpox. It is difficult to directly compare our findings and estimates for initial R, the initial effective reproduction rate for smallpox, to these other studies. Two of the studies do not use contemporary western outbreaks. One estimates the basic reproductive rate, (R 0, a theoretical parameter defined as the expected number of new infected hosts that an infectious host will produce in a large randomly mixed population of susceptible individuals [9] ), rather than effective reproduction rate. Specifically, Gani and Leach used epidemic modelling of data from 19th Century European smallpox outbreaks to estimate a R 0 of 3.5 to 6.0 [19] . Eichner and Dietz obtained an R of 6.9 from data on a 1967 outbreak in Nigeria [5] . Meltzer et al examined post-1961 outbreaks from countries around the world in estimating an R of 3 [8] .