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.