Smallpox infection occurs as major and minor form (variola maior, variola minor). The major form has 3 clinical phases: 1) the incubation period, 2) a prodromal illness, followed by 3) a fulminant infection [63] . The asymptomatic period lasts from 7 to 17 days (usually 12 to 14 days) after the initial exposure [62] . Asymptomatic viremia develops 3 to 4 days after infection. After multiplication of the virus in the spleen, bone marrow, and lymph nodes, a secondary viremia develops on approximately day 8 of infection [62] . During this prodromal phase, nonspecific symptoms, such as malaise, headache, backache, myalgias, fever, and vomiting develop. The overt smallpox syndrome occurs 2 to 3 days later, while the prodromal symptoms are subsiding. Infected leucocytes transport the virus to dermis and oropharyngeal mucosa, leading to the characteristic skin lesions [62] . Within 2 to 3 more days, a maculopapular rash appears; the greatest concentration of the lesions is in the face and distal extremities. The rash spreads from there in a centrifugal pattern [8] . Macules transform to papules to vesicles to pustules, each stage lasting 1 to 2 days. Vesicles and pustules are deep-seated, firm, round, well-circumscribed lesions; they are sharply raised and feel like small round objects embedded under the skin. Eventually, the lesions crust over and form scabs, leaving deep pitting scars that are unique to variola. Unlike varicella, all smallpox lesions are at the same stage of development.