An acute infectious viral disease characterized by severe systemic involvement and a single crop of skin lesions that proceeds through macular, papular, vesicular, and pustular stages. Smallpox is caused by variola virus, a brick-shaped, deoxyribonucleic acid-containing member of the Poxviridae family. Strains of variola virus are indistinguishable antigenically, but have differed in the clinical severity of the disease caused. Following a 13-year worldwide campaign coordinated by the World Health Organization (WHO), smallpox was declared eradicated by the World Health Assembly in May 1980. Smallpox is the first human disease to be eradicated.
Humans were the only reservoir and vector of smallpox. The disease was spread by transfer of the virus in respiratory droplets during face-to-face contact. Before vaccination, persons of all ages were susceptible. It was a winter-spring disease; there was a peak incidence in the drier spring months in the Southern Hemisphere and in the winter months in temperate climates. The spread of smallpox was relatively slow. The incubation period was an average of 10–12 days, with a range of 7–17 days. Fifteen to forty percent of susceptible persons in close contact with an infected individual developed the disease.
There were two main clinically distinct forms of smallpox, variola major and variola minor. Variola major, prominent in Asia and west Africa, was the more severe form, with widespread lesions and case fatality rates of 15–25% in unvaccinated persons, exceeding 40% in children under 1 year. From the early 1960s to 1977, variola minor was prevalent in South America and south and east Africa; manifestations were milder, with a case fatality rate of less than 1%.
There is no specific treatment for the diseases caused by poxviruses. Supportive care for smallpox often included the systemic use of penicillins to minimize secondary bacterial infection of the skin. When lesions occurred on the cornea, an antiviral agent (idoxuridine) was advised.
Edward Jenner, a British general medical practitioner who used cowpox to prevent smallpox in 1796, is credited with the discovery of smallpox vaccine (vaccinia virus). However, the global smallpox eradication program did not rely only on vaccination. Although the strategy for eradication first followed a mass vaccination approach, experience showed that intensive efforts to identify areas of epidemiologic importance, to detect outbreaks and cases, and to contain them would have the greatest effect on interrupting transmission. In 1978, WHO established an International Commission to confirm the absence of smallpox worldwide. The recommendations made by the commission included abandoning routine vaccination except for laboratory workers at special risk. See Animal virus, Vaccination