In smallpox, fever is present for 2 to 4 days before the rash begins, while with chickenpox, fever and rash develop at the same time.
All the pocks of the smallpox rash are in the same stage of development on any given part of the body and develop slowly. In chickenpox, the rash develops more rapidly, and vesicles, pustules, and scabs may be seen at the same time.

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