After a person has been infected through exposure to the smallpox virus, a symptom-free period follows. This is called the incubation period. It is normally 12 days in length but may be as short as 7 days or as long as 17 days.
The illness begins with fever. The patient feels sick and has symptoms such as head-ache and severe backache.
A rash appears after 2 to 4 days and progresses through characteristic stages of papules, vesicles, pustules and finally scabs. The scabs fall off at the end of the 3rd or 4th week.

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