The deadly viruses that vanished without trace

It was the year 1002. The English king Ethelred II – not-so-fondly remembered as “Æthelred the unready” – was at war. For over a century, Viking armies had been scoping out the land as a potential new home, under the command of leaders with well-groomed facial hair and evocative names, such as Swein Forkbeard.

So far, the Vikings had found the English resistance enticingly weak. But Ethelred had decided to make a stand. On 13 November, he ordered for every Danish man in the country to be rounded up and killed. Hundreds perished, and the incident went down in history as the St Brice's Day massacre. Ethelred’s brutal act proved to be in vain, and eventually most of England was ruled by Forkbeard’s son.

But what was a bad day to be a Viking in England was a gift for modern archaeologists. Over a thousand years later, 37 skeletons – thought to belong to some of the executed victims – were discovered on the grounds of St John’s College in Oxford. Buried with them was a secret.

When scientists analysed DNA from the remains earlier this year, they found that one of the men had been doubly unfortunate. Not only was he violently murdered – at the time, he had been suffering from smallpox.

And there was another surprise. This wasn’t the smallpox virus that we’re familiar with from recent history – the kind that was famously driven to extinction in the 1970s by a determined vaccination programme. Instead, it belonged to a remarkably different strain, one which was previously unknown, and silently disappeared centuries ago. It seems that smallpox went extinct twice.