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Causative Agent

Variola virus, a species of Orthopoxvirus

Incubation Period

12-14 days (range 7-17 days)

Infectious Period

From fever onset (usually 2–4 days before rash) until last scab has separated; about three weeks.


Aerosols/droplets from nasopharyngeal lesions and contact with contaminated articles.


Last naturally acquired human case in the world occurred in Somalia in 1977; global eradication was certified two years later.

There are at least 2 strains, variola major and the variola minor.

  • Variola major: the more severe form with case fatality rate up to 30-50% in susceptible populations.
  • Variola minor: milder form of the disease with more diminutive pox lesions; case fatality rate of 1-2% in susceptible populations.

  • Characteristic rash appears 2-4 days after non-specific, flu-like prodrome (fever and headache).
  • Maculopapular rash begins on mucosa of mouth and pharynx, face, hands, forearms and spreads to legs and centrally to trunk; lesions are more predominant on the face and extremities than on the trunk (centrifugal).
  • Lesions progress synchronously on any given part of the body from macules to papules to vesicles to pustules and to crusty scabs.
  • Two rare forms of invariably fatal smallpox have been reported:
    • Purpura variolosa or hemorrhagic type smallpox
    • Flat type smallpox

Differential diagnosis

  • Chickenpox, monkeypox, disseminated herpes zoster.
  • Clues to distinguish smallpox from chickenpox:
    • Smallpox lesions are synchronous in their stage of development.
    • Smallpox has many more lesions on the face and extremities than trunk (centrifugal spread)
    • Smallpox lesions are more common on palms and soles
    • Smallpox lesions are more deeply imbedded in the dermis compared with the superficial lesions of chickenpox

  • Electron microscopy, PCR, viral isolation (culture of pharyngeal swab or lesions).
  • Guarnieri bodies on Giemsa or modified silver stain.

  • Supportive care.
  • Antibiotics may be used for secondary bacterial infection.

Vaccination within 3 days of exposure may significantly ameliorate or prevent smallpox. Vaccination 4 to 7 days after exposure likely still offers some protection or modification of disease severity.

Notify MOH immediately on suspicion. Call MOH Communicable Diseases Surveillance team at: 98171463

  • Airborne and contact precautions. Isolate patients in negative pressure isolation room.
  • Patients should be considered infectious until all scabs separate and should be isolated during this period.
  • Droplet and airborne precaution for a minimum of 17 days following exposure for all persons in direct contact with the index case.


  1. Henderson DA, Inglesby TV et al. Smallpox as a biological weapon: medical and public health management. Working group on Civilian Biodefense. JAMA 1999; 281: 2127-37
  2. Committee on Epidemic Diseases. Clinical guidelines on anthrax, botulism, plague and smallpox. Epidemiological News Bulletin. 2001; 27:61-68
  3. CDC             (Atlanta),            Smallpox            overview.             2004.            Available             from Accessed Dec 2010.
  4. WHO. Smallpox. 2010. Available from Accessed Dec 2010.

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