The best available evidence stems from a series of studies by Sir Leonard Rogers (1868 Rogers ( -1962 [16] , who conducted epidemiologic surveys of smallpox outbreaks in India over a long period of time [17] [18] [19] . He also conducted a similar survey in England and Wales [20] . By analyzing the monthly mortality data from the late 19th to the early 20th century in these countries, Rogers argued that the smallpox epidemic in India is relatively uniform (i.e., not apparently cyclical) compared to that in England and Wales [17, 19, 21] . Further, he descriptively and implicitly suggested that there is a negative correlation between humidity and smallpox mortality, but there was little association between smallpox and rainfall [17, 18] . This effort was followed by Russell and Sundararajan [22] who supported the notion that a dry environment offers favorable conditions for smallpox transmission. These consistent findings have also been reported during the Smallpox Eradication Program (SEP), where a peak of smallpox incidence occurred from December to January in the Northern hemisphere (e.g., Indonesia and Bangladesh) and from May to June in the Southern hemisphere (e.g., Brazil) [23] [24] [25] [26] . However, the observed data during the SEP were greatly modified by intensive immunizations, and perhaps because of this, epidemics in other locations were not suggested to be seasonal [26] [27] [28] , leaving this issue yet to be clarified.