In the second half of the 20th century, the World Health Organization spearheaded an ultimately successful effort to eradicate a nemesis of humankind: smallpox.
The disease, caused by two variants of the variola virus, had ravaged civilizations for thousands of years, as evidenced by medical writings from ancient India and even by the mummy of Ramses V.
Over the centuries, it had struck populations in Africa, Europe, Asia and the Americas, killing hundreds of millions. Those fortunate enough to have survived frequently bore permanent scarring on their faces.
But variola possessed a fatal flaw — in nature, the virus infected only humans. With a vaccine originally developed by Edward Jenner in 1796, health authorities were able to launch mass inoculation campaigns across the world following World War II.