Many (six out of eleven) of the hospital outbreaks were only found in the above-mentioned additional outbreaks [20, 27, 29, 30, 38, 39] . Hospital outbreaks had higher initial reproductive rates than either community or mixed outbreaks, and had more cases and deaths than did community types. Moreover, the interaction of identification and setting in the studied outbreaks reveals a pattern similar to that seen in frequently unrecognized infectious diseases, such as tuberculosis and, more recently, SARS [42] . When index cases were identified, control is established early and both the median initial R and the number of generations were similarly low in all 3 settings. When the index cases were unidentified, a large increase in the median initial R was seen only in the hospital setting and a large increase in the number of generations was seen only in community and mixed settings. This is consistent with the observation that, within the close quarters of an institution, the transmission of smallpox is relatively effective before identification and containment measures are quite effective after identification. This impression is reinforced by examination of the two shipboard out-breaks that were excluded from the analysis. In these outbreaks, the initial R's were 6 and 25, respectively, but recognition was followed by control within two generations [22] .