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Recent discussions on the use of variola virus by bioterrorists have rekindled interest in the parameters that govern the transmissibility of smallpox. Here, the authors estimate by maximum likelihood the parameters of the spread of smallpox from historical data on an epidemic in 1967 in the town of Abakaliki, Nigeria, afflicting a religious group that refused vaccination. According to the authors' estimates, 79.9% (95% confidence interval (CI): 63.6, 87.9) of the infectious contacts occurred within the compounds of the cases and 93.3% (95% CI: 80.6, 98.8) among compound members and other close contacts. Each case had 0.164 (95% CI: 0, 1.31) sufficiently close contacts on average during the fever period that preceded the rash and 6.87 (95% CI: 4.52, 10.1) sufficiently close contacts during the whole course of infectivity. These results support the widely held belief that smallpox spreads slowly, mainly among close contacts, and that infectivity before the onset of rash was negligible. [ABSTRACT FROM AUTHOR]