To better understand its evolutionary history, scientists extracted the DNA, from partial mummified remains of a Lithuanian child, interred in the crypt of a church in Vilnius, believed to have died between 1643 and 1665, a period in which several smallpox outbreaks were documented throughout Europe with increasing levels of mortality. Researchers compared the 17thC strain to those from a databank of samples dating from 1940 up to its eradication in 1977. Surprisingly, the results shows that the evolution of smallpox virus occurred far more recently than previously thought, with all the available strains of the virus having an ancestor no older than 1580 AD.
The pox viral strains, that represent the true reservoir for human smallpox, remains unknown to this day. Camelpox is very closely related, but is not regarded as the likely ancestor to smallpox, suggesting that the real reservoir remains at large or has gone extinct.
One form, Variola major, was highly virulent and deadly, the other Variola minor more benign. However, the two forms experienced a ‘major population bottleneck’ with the rise of immunization efforts.The date of the ancestor of the minor strain corresponds well with the Atlantic Slave trade which was likely responsible for partial worldwide dissemination.
This raises important questions about how a pathogen diversifies in the face of vaccination. While smallpox is now eradicated in humans, we should remain vigilant about its possible reemergence until we fully understand its origins.
 Duggan et al: 17th Century Variola Virus Reveals the Recent History of Smallpox in Current Biology – 2016. See here.
 Smithson et al: Prediction of steps in the evolution of variola virus host range in PLoS One - 2014