Emerging and reemerging influenza virus infections.

Influenza A virus infection occurs in many species. Wild waterfowl harbor the widest variety of influenza A viruses and serve as a constant reservoir for the emergence of new viruses. Highly pathogenic avian influenza, or "fowl plague," has been a known poultry disease for more than 130 years. It continues to emerge and reemerge, but global changes in trade and poultry production have expanded the impact and geographic range of these outbreaks. One subtype of highly pathogenic avian influenza, H5N1, has infected poultry on several continents as well as many people, leading to a human disease that is markedly different from seasonal influenza and that is associated with high mortality.