Here’s why: by Paul Offit (thanks to Ed M.)
In May 1997, a 3-year-old boy died in Hong Kong of influenza. His death wasn’t unusual. Every year in every country in every corner of the world healthy children die from the disease. But this infection was different; health officials couldn’t figure out what type of influenza virus had killed the boy. The CDC sent a team of scientists to Hong Kong to investigate. Standing in a wet market, where local farmers slaughtered and sold their chickens, they found the source of the deadly virus.
The H5 strain of influenza virus that infected birds in Southeast Asia—named for the type of hemagglutinin on the viral surface—was particularly deadly, killing seven of every ten chickens. On December 30, 1997, health officials, to control the outbreak of bird flu before it spread to more people, slaughtered more than a million chickens. But the virus continued to spread. Bird flu attacked chickens in Japan, Vietnam, Laos, Thailand, Cambodia, China, Malaysia, and Indonesia. Then, to the horror of local physicians, the virus infected 18 more people, killing six: a death rate of 33 percent. (Typically, influenza kills fewer than two percent of its victims.) Soon the virus disappeared. Officials waited for an outbreak the following year, but none came. And it didn’t come the year after that or the year after that.
In late 2003, six years after the initial outbreak, bird flu reappeared in Southeast Asia. This time health officials found the virus even harder to control. Again, the virus first infected chickens. Officials responded by slaughtering hundreds of millions of them. Despite their efforts, bird flu spread from chickens to ducks, geese, turkeys, and quail. Then the virus spread to mammals: first to mice, then to cats, then to a tiger in a Thai zoo, then to pigs, then to humans. By April 2005, bird flu had infected 97 people and killed 53: a death rate of 55 percent. (continued)