ABOVE: Bird flu was thought only to infect birds until the first human cases were seen in Hong Kong in 1997. As of January 9, 2006, the World Health Organization had confirmed 146 cases of H5N1 in humans in Indonesia, Vietnam, Thailand, Cambodia, China and Turkey, leading to 76 deaths. BELOW: In 1918 the “Spanish flu” killed 50 million people. A pandemic today could be even more devastating.

Dr. Alexander Shestopalov, a Russian virologist, takes samples from ducks in Western Mongolia. He’s tracking flu along the Siberian flyway; from here, birds migrate to points around the globe—carrying deadly influenza with them.