Smallpox
is a viral infection that was eliminated from the world in
1977. It is caused by the variola virus. The incubation period
is about 12 days (range: 7 to 17 days) following exposure.
Initial symptoms include high fever, fatigue, and head and
back aches. A characteristic rash, most prominent on the face,
arms, and legs, follows in 2-3 days. The rash starts with
flat red lesions that evolve at the same rate. Lesions become
pus-filled and begin to crust early in the second week. Scabs
develop and then separate and fall off after about 3-4 weeks.
The majority of patients with smallpox recover, but death
occurs in up to 30% of cases.
Smallpox is spread from one person to another by infected
saliva droplets that expose a susceptible person having face-to-face
contact with the ill person. Persons with smallpox are most
infectious during the first week of illness, because that
is when the largest amount of virus is present in saliva.
However, some risk of transmission lasts until all scabs have
fallen off.
Routine vaccination against smallpox ended in 1972. The level
of immunity, if any, among persons who were vaccinated before
1972 is uncertain; therefore, these persons are assumed to
be susceptible.
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