On 9 December, 1979, health officials declared smallpox as the first and only human disease to be eradicated in what is considered the greatest achievement of modern medicine. Four decades on, the U.S. and Russia still maintain samples of the potentially deadly virus, and the debate on whether they should be kept or destroyed rages on.
We don’t know where smallpox came from. But the infection—which is caused by two related variola viruses—is thought to date back to the Egyptian Empire in the 3rd Century BCE, with its pustules found on the head of the Egyptian Pharaoh Ramses V. Trade and the expansion of civilizations helped the disease, found only in humans, spread. Characterized by symptoms including a fever, a widespread rash of fluid-filled blisters, vomiting and diarrhea, smallpox is estimated to have killed as many as 300 million people in the 20th century alone.