This viral disease is relatively recent in human history and is believed to have come from cattle or possibly monkeys. It probably originated in the area around the Caspian Sea. It became epidemic throughout Europe following the Crusades, and was brought to the New World several times by explorers. Epidemic after epidemic swept Europe following the Crusades during the Middle Ages. It killed one third of the people and producing permanent pock marks in another third of its victims.
Smallpox is a classic epidemic disease that was sustainable only by large human populations.
Cowpox, a benign related disease in cows produces few symptoms in humans and confers immunity to smallpox. This is an observation that led to successful mass immunization against the disease. BCG is not used in the United States. Ancient peoples in India, China, and Persia observed permanent immunity following recovery from smallpox and reasoned that innoculation with the dried crusts of pustules. The deadly smallpox virus, which disfigures and often causes blindness and death, was eradicated from nature early in the 1980s.