The threat posed by avian influenza appears to be rising, yet global and national health programs are preparing only fitfully. A lethal form of avian flu has rooted itself deeply into the poultry flocks of poor Asian countries that will have a hard time eradicating it. Every so often a sick bird infects a human, who usually dies from the encounter, and on rare occasions the virus seems to have spread from one person to another before the chain of infection dies out. All it would take to set off a raging global pandemic would be for the virus to mutate into a form that is readily transmissible among humans.1