The scientific knowledge in this field is continually evolving and improving, and the UK will therefore continue to review and refine its assessment of the evidence base. Smallpox and Season: Reanalysis of Historical Data Seasonal variation in smallpox transmission is one of the most pressing ecological questions and is relevant to bioterrorism preparedness. The present study reanalyzed 7 historical datasets which recorded monthly cases or deaths. In addition to time series analyses of reported data, an estimation and spectral analysis of the effective reproduction number at calendar time t, R(t), were made. Meteorological variables were extracted from a report in India from 1890–1921 and compared with smallpox mortality as well as R(t). Annual cycles of smallpox transmission were clearly shown not only in monthly reports but also in the estimates of R(t). Even short-term epidemic data clearly exhibited an annual peak every January. Both mortality and R(t) revealed significant negative association (P < .01) and correlation (P < .01), respectively, with humidity. These findings suggest that smallpox transmission greatly varies with season and is most likely enhanced by dry weather. Smallpox is the only disease to have been eradicated worldwide [1] . Despite the success story of vaccination and other public health interventions, the number of susceptible individuals has grown to date following cessation of routine vaccination, and the threat of bioterrorist attack has led to debates on countermeasures in such an event [2] . Various mathematical studies have been conducted as part of a preparedness program, including large-scale simulation of a bioterrorist attack and the public health countermeasures against it [3] [4] [5] [6] . Theoretical studies on the spread of smallpox include not only simulations but also quantitative analysis of historical data [7] [8] [9] [10] . A statistical modeling study suggests that a small outbreak could be contained only implementing contact tracing and isolation [11] . Moreover, those who underwent vaccination in the past are believed to be still protected against severe and fatal manifestations of smallpox even today [7, 12] .