The history of smallpox extends into pre-history. Genetic evidence suggests that the smallpox virus emerged 3,000 to 4,000 years ago. Prior to that, similar ancestral viruses circulated, but possibly only in other mammals, and possibly with different symptoms. Only a few written reports dating from about 500 AD to 1000 AD are considered reliable historical descriptions of smallpox, so understanding of the disease prior to that has relied on genetics and archaeology. However, during the 2nd millennium AD, especially starting in the 16th century, reliable written reports become more common. The earliest physical evidence of smallpox is found in the Egyptian mummies of people who died some 3,000 years ago. Smallpox has had a major impact on world history, not least because indigenous populations of regions where smallpox was non-native, such as the Americas and Australia, were rapidly and greatly reduced by smallpox (along with other introduced diseases) during periods of initial foreign contact, which helped pave the way for conquest and colonization. During the 18th century the disease killed an estimated 400,000 Europeans each year, including five reigning monarchs, and was responsible for a third of all blindness. Between 20 and 60% of all those infected—and over 80% of infected children—died from the disease.
During the 20th century, it is estimated that smallpox was responsible for 300–500 million deaths. In the early 1950s an estimated 50 million cases of smallpox occurred in the world each year. As recently as 1967, the World Health Organization estimated that 15 million people contracted the disease and that two million died in that year. After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the global eradication of smallpox in December 1979. Smallpox is one of two infectious diseases to have been eradicated, the other being rinderpest, which was declared eradicated in 2011.
Eurasian epidemics
It has been suggested that smallpox was a major component of the Plague of Athens that occurred in 430 BCE, during the Peloponnesian Wars, and was described by Thucydides.
Galen's description of the Antonine Plague, which swept through the Roman Empire in 165–180 CE, indicates that it was probably caused by smallpox. Returning soldiers picked up the disease in Seleucia (in modern Iraq), and brought it home with them to Syria and Italy. It raged for fifteen years and greatly weakened the Roman empire, killing up to one-third of the population in some areas. Total deaths have been estimated at 5 million.
A second major outbreak of disease in the Roman Empire, known as the Plague of Cyprian (251–266 CE), was also either smallpox or measles. The Roman empire stopped growing as a consequence on these two plagues, according to historians like Theodore Mommsen. Although some historians believe that many historical epidemics and pandemics were early outbreaks of smallpox, contemporary records are not detailed enough to make a definite diagnosis.
Around 400 CE, an Indian medical book recorded a disease marked by pustules and boils, saying "the pustules are red, yellow, and white and they are accompanied by burning pain … the skin seems studded with grains of rice." The Indian epidemic was thought to be punishment from a god, and the survivors created a goddess, Sitala, as the anthropomorphic personification of the disease. Smallpox was thus regarded as possession by Sitala. In Hinduism the goddess Sitala both causes and cures high fever, rashes, hot flashes and pustules. All of these are symptoms of smallpox.
Most of the details about the epidemics are lost, probably due to the scarcity of surviving written records from the Early Middle Ages. The first incontrovertible description of smallpox in Western Europe occurred in 581 CE, when Bishop Gregory of Tours provided an eyewitness account describing the characteristic symptoms of smallpox. Waves of epidemics wiped out large rural populations.
In 710 CE, smallpox was re-introduced into Europe via Iberia by the Umayyad conquest of Hispania.
The Japanese smallpox epidemic of 735–737 is believed to have killed as much as one-third of Japan's population.
The clearest description of smallpox from pre-modern times was given in the 9th century by the Persian physician, Muhammad ibn Zakariya ar-Razi, known in the West as "Rhazes", who was the first to differentiate smallpox from measles and chickenpox in his Kitab fi al-jadari wa-al-hasbah (The Book of Smallpox and Measles).