In human history, few things happen only once. Over millennia, even statistically rare events repeat. Yet despite huge efforts to replicate the feat, just once have we eradicated a human disease: smallpox, responsible for over 500 million deaths in the 20th century alone. Eradicating smallpox was a technological and a human challenge, like many of our toughest global problems (see "Why We Can't Solve Big Problems," p. 26).
展开▼
