Human influenza occurs worldwide, is endemic in tropical regions and occurs year round. In the temperate and subarctic regions, influenza in humans and swine is a 'winter disease'. The occurrence of influenza in winter is not well understood and may be associated with crowding, low humidity, and increased virus survival. Avian influenza viruses are prevalent in the fall of the year in wild ducks when the young congregate prior to migration and in shore-birds in the US in spring.
Equine, swine and avian influenza viruses have a wide geographic distribution; the avian influenza viruses from Europe and Asia can be separated phylogenetically from those in the Americas.
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