Chicken flu virus raises concerns.

The unusual influenza virus that killed an Asian boy last spring has reemerged and infected three more people, killing one.

In May, a boy living in Hong Kong contracted a unique strain of flu--apparently from chickens--and died of complications. Public health officials were alarmed because it was the first such infection reported in humans, who have no antibodies against the virus.

This appeared to be an isolated case until the three others surfaced in Hong Kong in November and December--independently of each other, it seems. A 54-year-old man died; two children survived.

In all four cases, the people probably caught the virus from chickens, not from each other, says Thomas W. Skinner of the federal Centers for Disease Control and Prevention (CDC) in Atlanta.

Although mild avian viruses have infected people in the past, causing eye irritation, this is the first direct chicken-to-human transmission of a virulent flu virus, says Virginia S. Hinshaw, a virologist at the University of Wisconsin-Madison.

"This one is different, and that's what's bothersome about it," she says.

Influenza viruses can kill chickens: Millions of them have died in southern China this year as a result of the new virus. Chicken flu outbreaks in Mexico and Pennsylvania in recent decades also killed millions of the birds, but no people.

Such avian viruses can spread to mammals, however. Ducks frequently pass flu viruses to pigs, which then act "like a laboratory mixing vessel," producing changes in the virus, says Dominick A. Iacuzio, a virologist at the National Institute of Allergy and Infectious Diseases in Bethesda, Md. Because there are billions of pigs, ducks, and chickens in China--many living in close proximity to people--novel flu strains often arise there.

The new virus is an influenza A subtype called H5N1. The letters stand for hemagglutinin and neuraminidase--two compounds that elicit an immune response. Three subtypes of hemagglutinin (H1, H2, and H3) and two of neuraminidase (N1 and N2) are recognized among flu viruses that have caused human disease. Having a ready response to these compounds helps a person ward off the disease, which is why the current flu vaccine includes two inactivated versions of influenza A virus--H1N1 and H3N2.

This season's flu shots offer no protection against H5N1, however, because it has a novel hemagglutinin. A new vaccine against H5N1 would take at least 6 weeks to develop, Hinshaw says. If H5N1 proves to be transmissible between people, the risks could be considerable.

The people infected appeared to be healthy before they caught the virus, Iacuzio says. Healthy people survive bouts with most flu viruses, but many young, elderly, and weakened people need the protection of a vaccine, which scientists modify to keep pace with each year's mutations.

Every so often, as in 1957 and 1968, a flu virus takes a sharp genetic turn, catching even healthy immune systems off guard and causing widespread infection and death. The worst flu pandemic, in 1918, was caused by a radically new strain.

Researchers who monitor these viruses find the Hong Kong strain unsettling. To be deadly, a flu virus needs three things: people who have never been exposed to it, enough virulence to cause deadly disease in people, and the ability to jump from person to person. "We have two out of three" in Hong Kong, Iacuzio says.

Some people doubt that the virus will spread widely. "In a place like Hong Kong, if it were transmitted rapidly [from person to person], there would be an explosion of people coming in sick," says Paul W. Ewald a biologist at Amherst (Mass.) College. So far, that hasn't happened.

Other scientists are less optimistic. The CDC has dispatched five epidemiologists to Hong Kong to obtain samples of the flu strain and to assess its transmissibility.