Studies on smallpox control have progressed in parallel with the development of epidemiological and statistical methods, and because of the eradication before maturation of biostatistics, many questions have remained in regards to the details of the epidemiology. Seasonal variation in smallpox transmission is one of the most pressing ecological questions playing a key role in determining the transmission dynamics, should a future outbreak occur following the deliberate release [1, 4] . For example, clarification of the seasonal preference of variola virus is crucial for identifying and forecasting the disease risk using ecological data [13] . Although seasonal occurrence of smallpox was documented early on among directly transmitted infectious diseases [14, 15] , and whereas the disease is believed to be one of the "winter diseases" in industrialized countries, even the presence of seasonality has not been investigated in detail.