The initial symptoms of smallpox typically manifest 12 to 14 days after exposure to the virus, during the incubation period when the individual feels healthy and is not contagious. This incubation period lasts between seven to 17 days.
Once the incubation period ends, flu-like symptoms suddenly appear, including fever, malaise, headache, severe fatigue, body and back pain, and gastrointestinal symptoms such as vomiting and diarrhea. Following these symptoms, a smallpox rash emerges a few days later. This rash begins as flat, red spots that progress into small blisters filled with clear fluid and later with pus, starting on the face, hands, and forearms before spreading to the trunk. Notably, the rash is most pronounced on the palms and soles and may involve the mucous membranes of the nose and mouth.
As the pustules develop, they can cause intense pain. After approximately eight to nine days, scabs form over the lesions, eventually falling off and leaving behind deep pitted scars. Unlike chickenpox, where lesions appear in crops, smallpox lesions progress uniformly through stages in a specific area.
If recovery does not occur, death typically happens during the second week of illness.
Smallpox can be distinguished from chickenpox by several factors:
- Severity and location of lesions: Smallpox lesions are deeper and occur more on the face, arms, and hands compared to chickenpox, which primarily affects the trunk.
- Types of lesions: Chickenpox lesions of all stages (scabs, vesicles, and pustules) are present simultaneously in a given area, while smallpox lesions in a specific area progress uniformly through stages.
- Timing of transmission: A person with chickenpox can transmit the virus before symptoms appear, while a person with smallpox becomes infectious only after signs and symptoms develop. Additionally, chickenpox remains contagious until scabs fall off, whereas smallpox is most contagious during the initial stages, after fever develops and during the first week of the rash.