Also known as variola. Smallpox is an acute contagious disease caused by the variola
virus, a member of the orthopoxvirus family. It was one of the world's most devastating
diseases known to humanity. It was declared eradicated in 1980 following a global
immunization campaign led by the World Health Organization.
Smallpox was transmitted from person to person via infective droplets during close
contact with infected symptomatic people. Vaccine administered up to 4 days after
exposure provided protective immunity and was preventing infection and lessening the
severity of the disease. The last known natural case was in Somalia in 1977. Since
then, the only known cases were caused by a laboratory accident in 1978 in
Birmingham, England, which killed one person and caused a limited outbreak.
Because of its high case-fatality rates and transmissibility and because people haven't
been vaccinated against it in years, smallpox now represents a serious bioterrorist
threat category A. The incubation period is about 12 days (range: 7-17 days) following
exposure. Initial symptoms include high fever, fatigue, headaches, and backaches. A
characteristic rash, most prominent on the face, arms, and legs, follows in 2 to 3 days.
The rash starts with flat red lesions that evolve in 2 to 3 days. Lesions become pusfilled
and begin to crust early in the second week. Scabs develop and then separate
and fall off after about 3 to 4 weeks. The majority of patients with smallpox recover, but
death occurs in up to 30 percent of cases. Smallpox is spread from one person to
another via infected saliva droplets as occurs during face to face contact. 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.