Every so often, a strain of influenza unfamiliar in humans suddenly begins passing from person to person. Because the virus is so unusual, few if any people have built-in immunity from past exposures. Even the vaccinated have no defense; flu shots shield against influenza variants that health experts have anticipated will be active in a given flu season, not against other, unforeseen kinds. Finding no deterrent, the new strain spreads unabated, causing illness--and death--on a global scale.

The worst worldwide epidemic, or pandemic, on record struck in 1918 and killed more than 20 million people, sometimes within hours after the first symptoms appeared. This disaster, traced to the so-called Spanish influenza virus, was followed by epidemics of Asian flu in 1957, Hong Kong flu in 1968 and Russian flu in 1977. (The names reflect popular impressions of where the pandemics began, although all four episodes, and perhaps most others, are now thought to have originated in China.)

   
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