Vikings had smallpox and may have helped spread the world's deadliest virus

Scientists have discovered extinct strains of smallpox in the teeth of Viking skeletons - proving for the first time that the killer disease plagued humanity for at least 1400 years. Smallpox spread via infectious droplets, killed around a third of sufferers and left another third permanently scarred or blind. Around 300 million people died from it in the 20th century alone before it was officially eradicated in 1980 -- the first human disease to be wiped out.