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.