In 1997 the world came perilously close to a global epidemic of the "flu." If this particular virus had attained the ability to spread from person to person, the pandemic might have taken the lives of a third of the human population. As it was, only six people died—and all of them had contracted the virus from chickens sold in Hong Kong poultry markets. The only thing that saved us was the quick thinking of scientists who convinced health authorities to slaughter more than a million domesticated fowl in the city's markets. The avian virus turned out to be a new strain—one that the human population had never seen before. Webster, one of the scientists who consulted Hong Kong authorities in 1997, and Walker tell a horrific tale: These new, deadly strains arise a few times every century, and the next one is around the corner.