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Transmission

Smallpox was highly contagious, but generally spread more slowly and less widely than some other viral diseases, perhaps because transmission required close contact and occurred after the onset of the rash. The overall rate of infection was also affected by the short duration of the infectious stage. In temperate areas, the number of smallpox infections was highest during the winter and spring. In tropical areas, seasonal variation was less evident and the disease was present throughout the year.^[33] Age distribution of smallpox infections depended on acquired immunity. Vaccination immunity declined over time and was probably lost within thirty years.^[34] Smallpox was not known to be transmitted by insects or animals and there was no asymptomatic carrier state.^[33]

Transmission occurred through inhalation of airborne variola virus, usually droplets expressed from the oral, nasal, or pharyngeal mucosa of an infected person. It was transmitted from one person to another primarily through prolonged face-to-face contact with an infected person.^[14]

Some infections of laundry workers with smallpox after handling contaminated bedding suggested that smallpox could be spread through direct contact with contaminated objects (fomites), but this was found to be rare.^[14][35] Also rarely, smallpox was spread by virus carried in the air in enclosed settings such as buildings, buses, and trains.^[32] The virus can cross the placenta, but the incidence of congenital smallpox was relatively low.