Killing around 300 million people in the 20th century alone, smallpox – the disease caused by the variola virus – is one of the deadliest diseases in history and the first to be officially eradicated. But it is now clear that humans have been plagued with smallpox for much longer than previously evidenced.

In the teeth of Viking skeletons unearthed from sites across Northern Europe, scientists have extracted new strains of smallpox that are poles apart from their modern descendants.

“The ancient strains of smallpox have a very different pattern of active and inactive genes compared to the modern virus,” Dr Barbara Mühlemann, of the University of Cambridge, said in a statement. “There are multiple ways viruses may diverge and mutate into milder or more dangerous strains. This is a significant insight into the steps the variola virus took in the course of its evolution.”

Smallpox is a disease spread from person to person via infectious droplets. The earliest genetic evidence of the disease found prior to this study dates back to the mid-1600s, but Mühlemann and her colleagues discovered extinct smallpox strains in 11 individuals located in Viking-era burial sites in Denmark, Norway, Sweden, Russia, and the UK, dated to nearly 1,400 years ago. In fact, the Viking way of life may have also helped to spread this disease.

“We already knew Vikings were moving around Europe and beyond, and we now know they had smallpox,” said Professor Eske Willerslev, also from the University of Cambridge. “People travelling around the world quickly spread Covid-19 and it is likely Vikings spread smallpox. Just back then, they travelled by ship rather than by plane.”

The new study, published in Science, has helped to shed light on the rather unclear history of the virus. Indeed, it is unknown how the virus first came to infect humans but, like Covid-19, it may come back to animals. “The early version of smallpox was genetically closer in the pox family tree to animal poxviruses such as camelpox and taterapox, from gerbils,” Dr Lasse Vinner, a virologist from The Lundbeck Foundation GeoGenetics Centre, Denmark, said.

Although it is not known if this strain was as deadly as the modern virus, which killed around one-third of its sufferers, knowledge of its existence over 1,400 years ago can help to protect us in the present.

“Smallpox was eradicated but another strain could spill over from the animal reservoir tomorrow,” Willerslev concluded. “What we know in 2020 about viruses and pathogens that affect humans today is just a small snapshot of what has plagued humans historically.”