Smallpox has been around since the Viking Age. But its pathogens were markedly different from the modern species of this virus.

Smallpox has plagued humanity for at least 1200 years, and the lethality of this pathogen has increased over time. Scientists came to such conclusions by sequencing the genomes of smallpox viruses found in the skeletons of ancient Vikings. An article about this was published in Science.

Smallpox, which was officially eradicated in the 1980s, is one of the deadliest diseases in human history. In the 20th century alone, about 300 million died from it. The smallpox virus killed about a third of those infected and left another third disfigured or blind. The introduction of a global vaccination program helped to stop its spread.

How long has this virus been circulating in the human population? According to some versions, people began to get sick with smallpox about 10 thousand years ago. But there is no scientific evidence that the virus-infected humans before the 17th century. Also, scientists do not yet have a unified theory about the origin of this pathogen.

The study of the remains of the Vikings who died about 1200 years ago made it possible to partially understand these issues. The oldest known strains of the smallpox virus were found in the teeth of the dead soldiers. The genetic information extracted from the remains reveals the veil of secrecy over the evolutionary development of the pathogen.

“The date of the emergence of smallpox has always been unclear, but thanks to the [genetic] sequence of the earliest known strains of the killer virus, we have proven for the first time that smallpox existed in the Viking Age,” says Professor Martin Sikora, University of Copenhagen, one of the study’s authors. “While we do not know for sure whether this strain of smallpox was fatal and whether it caused the death of the Vikings [whose remains] we collected, they certainly died with smallpox in their bloodstream”.

In total, smallpox was found in 11 skeletons of Vikings from different parts of Europe – they were found in Denmark, Great Britain, Sweden, Norway, and Russia. Scientists were able to recover almost complete genomes of viruses for four samples. The viruses found in the remains of the warriors were not the same as 20th-century smallpox, the researchers said. The pathogens that infected the Vikings were genetically closer to animal viruses such as camelpox or the small-footed gerbils. It is likely that the ancient variola was not nearly as lethal as the modern one.

The discovery also shows that already 1200 years ago, smallpox was widespread in Northern Europe. This refutes theories that smallpox was introduced to Europe much later – for example, soldiers returning from the Crusades of the XI-XIII centuries.

The work of scientists makes it possible to trace how smallpox viruses have changed over time and will prevent new outbreaks of viral infections. “Knowledge of the past can protect us today. When an animal or plant dies out, it does not re-emerge. But mutations can reappear, and viruses can mutate or be transmitted [to humans] from animals, ”sums up the head of the study, Eske Villerslev.