Smallpox was endemic to Europe, Asia and Arabia for centuries, a persistent menace that killed three out of ten people it infected and left the rest with pockmarked scars.
But the death rate in the Old World paled in comparison to the devastation wrought on native populations in the New World when the smallpox virus arrived in the 15th century with the first European explorers.
The indigenous peoples of modern-day Mexico and the United States had zero natural immunity to smallpox and the virus cut them down by the tens of millions.
“There hasn’t been a kill off in human history to match what happened in the Americas—90 to 95 percent of the indigenous population wiped out over a century,” says Mockaitis. “Mexico goes from 11 million people pre-conquest to one million.”
Centuries later, smallpox became the first virus epidemic to be ended by a vaccine. In the late 18th-century, a British doctor named Edward Jenner discovered that milkmaids infected with a milder virus called cowpox seemed immune to smallpox.
Jenner famously inoculated his gardener’s 8-year-old son with cowpox and then exposed him to the smallpox virus with no ill effect.
“[T]he annihilation of the smallpox, the most dreadful scourge of the human species, must be the final result of this practice,” wrote Jenner in 1801.
And he was right. It took nearly two more centuries, but in 1980 the World Health Organization announced that smallpox had been completely eradicated from the face of the Earth.