Smallpox is a contagious disease and kills as many as 30% of those infected. The incubation period of smallpox is usually 12 to 14 days during which there is no evidence of the disease. During this period, the person looks and feels healthy and cannot infect others.
The incubation period is followed by the sudden onset of flu-like symptoms. Two to 3 days later, the temperature falls and the patient feels somewhat better, at which time the characteristic rash appears, first on the face, hands and forearms and after a few days it progresses to the trunk.
Smallpox is transmitted from person to person by infected aerosols and air droplets spread through face-to-face contact with an infected person after fever has begun. The disease can also be transmitted via contaminated clothes and bedding, though the risk of infection from this source is much lower.
In the absence of immunity induced by vaccination, humans appear to be universally susceptible to infection with smallpox. Vaccine administered up to 4 days after exposure to the virus, and before the rash appears, provides protective immunity and reduces the severity of the disease.