Bird flu persists in Asia, and that worries health officialsworldwide. The H5N1 virus is so lethal that it is believedthat it could kill fifty million people if it mutates andbecomes transmissible from human to human.
In 1997, virologists did not believe that the virus could betransmitted from birds to humans, but the deaths of 6 of 18people who got the illness in Hong Kong that year provedthem wrong.
At the time, the virus could only infect birds, but it hasbeen mutating at exceptional speed, and now is known to beable to infect cats, pigs and tigers.
The pathogen's ability to infect pigs is especiallyworrisome, because of the metabolic similarities betweenpigs and humans, and the known tendency of swine infections,such as the swine flu epidemic of 1976, to jump to the humanpopulation.
Asian countries such as Thailand and Vietnam, where bird fluis persistent, are killing hundreds of thousands of infectedbirds and other animals, and tracking all human cases of thedisease to be certain that the virus has not become capableof traveling from human to human with no animal host.
World Health Organization officials privately expressconcern that the movement of bird flu into the humanpopulation is inevitable. "Almost everyone in the fieldfeels that an influenza pandemic is virtually inevitable,and that we need to be prepared for it," said Stephen Morse,the director of the Center for Public Health Preparedness atColumbia University in New York.
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