Smallpox most likely originated in Africa, evolving from a virus that infected rodents before making the jump to humans thousands of years ago. The exact timing is debated, with genetic estimates ranging from roughly 16,000 to 68,000 years ago depending on which historical records researchers use to calibrate their models. What’s clear is that the virus, known scientifically as Variola, belongs to a family of related poxviruses that circulate in animals, and at some point it adapted so thoroughly to humans that we became its only host.
An Animal Virus That Jumped to Humans
Variola virus didn’t appear out of nowhere. It belongs to a group called orthopoxviruses, which includes monkeypox and camelpox. Genetic analysis shows that Variola’s closest relatives are taterapox virus, found in African gerbils, and camelpox virus, which infects camels. All three cluster tightly together on evolutionary trees, suggesting they share a common ancestor.
The leading theory, based on these relationships, is that Variola evolved from a virus circulating in African rodents. At some point, that ancestral virus spilled over into humans and gradually adapted to spread exclusively between people. This is a pattern seen across infectious diseases: a pathogen that thrives in an animal population occasionally jumps species, and if conditions are right, it evolves to transmit efficiently in the new host. In Variola’s case, the adaptation was so complete that no animal reservoir remained. Humans were the only species that carried the virus, which is ultimately what made eradication possible.
What Ancient DNA Reveals
For most of history, researchers relied on written records and physical evidence like mummy scarring to trace smallpox’s past. That changed dramatically in 2020, when a team recovered Variola DNA from the remains of 13 people buried across northern Europe. Eleven of those individuals dated to roughly 600 to 1050 CE, overlapping the Viking Age. Four yielded near-complete viral genomes.
These sequences were remarkable for two reasons. First, they pushed confirmed smallpox cases back by about 1,000 years earlier than previous physical evidence had established. Second, the Viking-era strains belonged to a now-extinct lineage, a sister group to all the modern Variola strains that circulated before eradication. The distinct patterns of gene changes in these ancient viruses show that different evolutionary paths led to versions of smallpox capable of spreading widely among humans. Based on this genetic data, the most recent common ancestor of all known Variola strains lived roughly 1,700 years ago, around the 4th century CE.
That doesn’t mean smallpox itself is only 1,700 years old. It means the virus diversified into the lineages we can detect around that time. The actual jump from animals to humans almost certainly happened much earlier, but ancient DNA degrades, and samples from deeper in history are extremely rare.
The Earliest Written Descriptions
The oldest written account of a disease matching smallpox appeared in China in the 4th century CE. Descriptions followed in India in the 7th century and in Asia Minor (modern-day Turkey) by the 10th century. These dates align roughly with what genetic evidence suggests about the virus diversifying and spreading across Eurasia, though the disease was almost certainly present in human populations before anyone wrote about it.
Some researchers have pointed to possible evidence of smallpox on Egyptian mummies, including lesions on the mummified remains of Pharaoh Ramses V, who died around 1157 BCE. This physical evidence is suggestive but difficult to confirm definitively, since other diseases can produce similar scarring.
Two Strains, Two Levels of Severity
Smallpox existed as two distinct forms. Variola major was the severe version, killing roughly 30 percent of those infected. Variola minor, sometimes called alastrim, caused a much milder illness with a fatality rate below 1 percent. These weren’t just clinical labels. They represent genuinely separate evolutionary lineages.
Genetic studies place the two strains into distinct branches on the Variola family tree. The Variola major lineage (called P1) included strains from across Africa, Asia, and Europe. The Variola minor lineage (P2) was restricted to West Africa and the Americas. Researchers estimate the P2 lineage’s common ancestor dates to roughly the 1860s, suggesting the milder strain emerged or diversified relatively recently in the virus’s long history. How exactly Variola minor arose, whether through a single mutation event or gradual genetic drift, remains an open question, but its geographic concentration in West Africa and the Americas points to a distinct evolutionary trajectory that unfolded over the final centuries before eradication.
How Smallpox Reached the Americas
Smallpox was absent from the Western Hemisphere until European contact. The first recorded pandemic in the Americas began in 1518 on the island of Hispaniola, from where it swept through the Caribbean, across Mexico, and into South America. The virus became a devastating force in the Spanish conquests of the Aztec and Inca empires, killing vast numbers of people who had no prior exposure and no immunity.
The toll was staggering. When a major epidemic hit coastal New England beginning in 1616, English observers described entire communities destroyed. Contemporary accounts reported that in some areas, 90 percent of the Indigenous population died. One merchant noted that where he had previously seen two or three hundred people, “scarce thirty” remained. The historian Sherburne Cook, adjusting for exaggeration in the primary sources, estimated that roughly 75 percent of the Massachusett and Pokanoket populations perished in that single epidemic. The 1616 to 1619 outbreak fundamentally transformed the demographic and political landscape of New England, clearing the way for English colonization of the region.
This pattern repeated across the Americas for centuries. Populations with no history of exposure to orthopoxviruses faced catastrophic mortality when the virus arrived, a direct consequence of smallpox’s long co-evolution with Eurasian and African populations.
The Virus Itself
Variola is a physically large virus, roughly 330 by 250 nanometers, with a distinctive brick-like shape visible under an electron microscope. Unlike most DNA viruses, which hijack a cell’s nucleus to replicate, poxviruses carry their own molecular machinery and replicate in the cell’s outer compartment. This self-sufficiency is part of what makes them unusual in virology.
The virus has a relatively large genome of about 186,000 base pairs of double-stranded DNA, with distinctive hairpin loops at each end. That size gave Variola plenty of genetic toolkit to evade the human immune system, which it did with extraordinary efficiency for thousands of years, until vaccination and a global eradication campaign ended its reign in 1980.