Great discoveries made in the mid-nineteenth century facilitated major advances in pox virology. Based on the report by Bollinger [5] on poxvirus infected cells in chickens, and subsequent work by Fenner and Burnet [19] , APVs and other poxviruses were classified on the basis of original host, growth and morphological characteristics in the CAM of embryonated eggs or cell cultures and on clinical manifestations in different diseases of humans, birds and animals [20] rather than on genetic identity, which may provide both rapid and reliable virus identification [21] [22] [23] . These criteria have remained the basis for subsequent classification of APVs despite development of new molecular tools that have the capability of resolving the issue of species specificity of APV.