This summer, a flu strain unlike any that has infected humans before appears to have jumped directly from birds to a human, killing a Hong Kong boy. The event rang alarm bells around the world, as public health experts feared that the virus might touch off a pandemic, because nobody would have immunity to this new strain. On 20 August, the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta dispatched researchers to Hong Kong to join an international team of scientists conducting an "extensive investigation" there and in mainland China.
Earlier this week, Hong Kong officials announced that the scare is over--for now. To date, says CDC epidemiologist Nancy Arden, no other human cases of infection with this strain, known as H5N1 ("H" stands for hemagglutinin and "N" for neuraminidase, both of which are surface proteins of the virus), have been found. But the researchers are still trying to figure out why this particular virus crossed the species barrier, and some fear that a real pandemic will be touched off by just such an event in the future.
The episode began in May, when scientists at Hong Kong's Department of Health isolated an influenza virus from the trachea of a 3-year-old boy who died from an acute respiratory illness. World Health Organization reference labs in three different countries identified the virus as H5N1, a typically avian virus. This strain was first isolated by the Department of Agriculture and Fisheries in Hong Kong after it killed 4500 chickens on three farms between March and early May, says the University of Hong Kong's Kennedy Shortridge, who first identified it.
"When we heard about this, it raised a lot of questions," says Dominick Iacuzio, an influenza specialist at the U.S. National Institute of Allergy and Infectious Diseases. Robert Webster, an influenza specialist at St. Jude Children's Research Hospital in Memphis, Tennessee, notes that "the dogma was H1, H2, and H3 are human viruses" because these hemagglutinins specifically bind to molecules on the surface of human cells. But the infection of a human with an H5 suggests that this tidy scenario is too simplistic.
Webster suspects that this case may have surfaced because of improved surveillance rather than a fundamental shift in this strain of flu's ability to infect human cells. Then again, he's seriously concerned that a similar virus sooner or later will get a foothold in humans. Both Webster and Iacuzio see this case as something of a fire drill for public health officials who battle influenza. Says Webster: "This provides good practice for what eventually will happen."
[A more detailed News story  will be published in the 12 September issue of Science.]