Final answer:
The smallpox rash typically appears on the legs last, following the usual progression from face to trunk and then to extremities in the course of the infection.
Step-by-step explanation:
The question pertains to the progression of the smallpox rash in individuals infected with the virus. Based on information about similar viral rash progressions, like chickenpox, the rash typically starts on the face. It then progresses to other parts of the body such as the chest and extremities, including the arms and legs, with the concentration most prevalent on the trunk. However, since the question specifically refers to smallpox, we must be cautious to not confuse it with chickenpox. Smallpox rashes are known to appear almost simultaneously on all parts of the body, but the evolutionary stages of the rash (from macules to papules to vesicles, pustules, and finally scabs) also take place last on the extremities.
Therefore, based on the known progression of smallpox, one could suggest that the rash would usually appear on the legs last. It is important not to mistake smallpox for chickenpox or other viral rashes, as smallpox was a different disease with its own distinct pattern of rash progression.