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The most frequent mode of transmission is from person to person spread via direct inoculation of infective droplets on to the oral, nasopharyngeal or respiratory mucosa during close contact with an infected person. From the mucosa the virus is transferred to local lymphoid tissue where replication occurs (1)

  • household contact produces the highest attack rate
    • in outbreaks in Asia and Africa, the attack rate in households varied from 37% to 96%,
  • casual contact, such as working in the same building, is much less likely to result in infection, although airborne spread of virus in draughts or air conditioning systems is known to cause transmission
  • contaminated clothing or bed linen can also spread the virus

Humans are the only natural hosts of variola. There is no known animal reservoir or vector for the smallpox virus (1).

The incubation period is usually 10 to 16 days (but can range from 7 to 17 days) with a median of 12 days. Patients are not infectious during the asymptomatic incubation period

  • they become infectious with the onset of fever.
  • infectiousness then increases until the onset of vesicular rash and remains high for the next 7 days.
  • patients remain infectious until the last scabs fall off
    • however the virus shed from the skin is not highly infectious and exposure to patients in the late stages of the disease is unlikely to produce infection in susceptible contacts

Contacts of cases can be classified according to their risk of infection:

  • category A contacts are those people who have had close contact with infectious cases and are at risk of infection from large droplets or contaminated fomites. They include household contacts and people who have spent significant periods in face-to-face contact with cases or who have had contact with heavily contaminated fomites
  • category B contacts are those people who have had less intense contact with infectious cases but are still at risk via aerosol transmission of virus. They include contacts at work and in social settings


  • as a precaution, for the purpose of contact tracing, patients should be regarded as infectious from 24 hours prior to the time when fever was first recognised.


Last reviewed 01/2018