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Quick Facts about Smallpox

Information adapted from the CDC and Johns Hopkins Center for Civilian Biodefense Center.

Signs and symptoms
Post-exposure prophylaxis

Signs and symptoms

  • Smallpox is caused by the variola virus
  • Incubation period is about 12 days (7 to 17 days)
  • Mode of transmission
    • Person to person by infected saliva droplets
    • May be spread by contaminated clothing or bedding
    • Most infectious during first week
    • Remain infectious until scabs fall off
  • Initial symptoms
    • Fever, fatigue, head and back aches
      • The absence of significant fever prodrome makes smallpox very unlikely.
    • 50% vomiting, 10% diarrhea
    • Characteristic rash within 2 -3 days
      • Begins on face, proximal arms and legs
      • Spreads to chest, distal extremities
        • Most concentrated on the face and distal extremities
      • At any one time, the rash is in the same stage of development on any one part of the body
      • Stages of rash
  • Flat, red rash followed in 2 days by
  • Papules followed in 4 -5 days by
  • Vesicles followed in 1 - 2 days by
  • Pustules followed in 7 - 14 days by
  • Crusts and scabs followed by
  • Scabs falling off and scarring (2 - 3 weeks)
  • Disease in previously vaccinated persons
    • Vaccination ended in 1972
    • Current level of immunity is unknown
    • Persons are assumed to be susceptible
  • Complications
    • Secondary bacterial infections of the skin
    • Keratitis and corneal ulceration
    • Viral arthritis and osteomyelitis
    • Bacterial pneumonia
    • Orchitis
    • Encephalitis
  • Differential diagnosis
    • Varicella (chickenpox)
      • Rash begins on trunk, spreads to face and extremities
      • Most lesions are concentrated on trunk or equally distributed between trunk and extremities
      • At any one point in time, rash in different stages
  • Diagnostic Tests
    • Clinical presentation
    • Virus culture
      • Skin lesions, oropharynx, conjunctiva, urine
    • Serological testing for antibody
      • Pared samples 2 - 3 weeks apart
      • Hemagluttination inhibition, compliment fixation, or gel precipitation
    • DFA and IFA can be used to diagnose varicella-zoster virus
  • Case fatality rate 30%


Post-exposure Prophylaxis

  • The vaccine lessens severity or prevents illness
  • Must be given within 4 days after exposure
  • Vaccine
    • Live vaccina virus
    • Does not contain smallpox virus



  • No antiviral treatment available
  • Antibiotics for secondary bacterial infections
  • Supportive care (fluids, pain, fever)
  • Mass vaccination for the general public is not recommended
    • Risk currently outweighs benefits
    • Sufficient vaccine not available


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