"The incubation period for smallpox ranges from 7-to-17 days (usually 12-to-14 days), which is relatively long for a viral illness. Initial symptoms consist of high fevers, malaise, headache, and backache, followed by the appearance of small raised red spots (maculopapular rash) that begin inside the mouth and on the face and forearms, then spread sequentially to the legs and trunk. The skin lesions evolve into small blisters, then pustules. Scabs develop, which eventually separate, leaving pitted scars. In its early stages, the rash may resemble that of chickenpox, with two key differences. (1) In smallpox, the lesions are densest on the face and outermost extremities, whereas in varicella, the greatest number of lesions are on the trunk. (2) In smallpox, all lesions are at the same stage of development, whereas in varicella, the lesions are at different stages of evolution, i.e. some may be blisters while others are scabbing."