section_path: Cause/Transmission
section_name: Transmission
paragraph_text: 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. Age distribution of smallpox infections depended on acquired immunity. Vaccination immunity declined over time and was probably lost within thirty years. Smallpox was not known to be transmitted by insects or animals and there was no asymptomatic carrier state.
  
  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.
  
  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. Also rarely, smallpox was spread by virus carried in the air in enclosed settings such as buildings, buses, and trains. The virus can cross the placenta, but the incidence of congenital smallpox was relatively low. Smallpox was not notably infectious in the prodromal period and viral shedding was usually delayed until the appearance of the rash, which was often accompanied by lesions in the mouth and pharynx. The virus can be transmitted throughout the course of the illness, but this happened most frequently during the first week of the rash when most of the skin lesions were intact. Infectivity waned in 7 to 10 days when scabs formed over the lesions, but the infected person was contagious until the last smallpox scab fell off.
  
  Concern about possible use of smallpox for biological warfare led in 2002 to Donald K. Milton's detailed review of existing research on its transmission and of then-current recommendations for controlling its spread. He agreed, citing Rao, Fenner and others, that “careful epidemiologic investigation rarely implicated fomites as a source of infection”; noted that “Current recommendations for control of secondary smallpox infections emphasize transmission ‘by expelled droplets to close contacts (those within 6–7 feet)’”; but warned that the “emphasis on spread via large droplets may reduce the vigilance with which more difficult airborne precautions are maintained”.