Scientists have found extinct strains of smallpox in the teeth of Viking skeletons, proving that the disease has plagued humanity for over a millennium.
Smallpox is spread from one person to another through infectious droplets.
It can kill as many as a third of those who contract it and leave another third permanently scarred or blinded as a result.
As many as 300 million people died from it in the 20th century alone, before it was completely eradicated in 1980 thanks to an international vaccination effort. This made it the first human disease ever to be wiped out.