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Some major diseases of wild birds have emerged dramatically in North America recently due in part to the rapid global movement of animals and pathogens and major changes in the size, quality, and continuity of natural habitats. These diseases include invasive pathogens, emergence of new disease agents or syndromes, and reemergence of some established diseases. A prominent example is West Nile virus that invaded North America, became easily established, and has expanded throughout the continent within 4 years. A number of factors have influenced the frequency of occurrence, the variety of species affected, and the geographical extent of these emergent diseases. Management and control is limited by the transitory nature of many bird species, difficulties in diagnosing the diseases, and the lack of adequate resources.