Influenzas are broadly divided into three types: A, B, and C. Type A influenza includes most human and all avian influenza (AI) viruses. AI is a viral disease that naturally infects waterfowl and some species of shorebirds. Influenza viruses are divided into subtypes based on the two proteins, hemaglutinin (H) and neuraminidase (N), that they have on their surfaces. There are 16 recognized H types, and 9 N types, and these are known to occur in a number of different combinations.

Avian influenza is broadly divided into highly pathogenic (HPAI) and low pathogenic (LPAI) strains based on its ability to cause disease in poultry. Low pathogenic AI is a natural infection of waterfowl that may cause minimal to no signs of disease in domestic poultry and wild birds. Highly pathogenic AI is rarely found in waterfowl, and causes severe disease in domestic poultry with a high case fatality rate (death rate). Only two types of AI viruses, H5 and H7, are known to include highly pathogenic viruses. It is important to understand, however, that not all H5 and H7 influenza viruses are highly pathogenic. Due to stringent biosecurity practices, AI is uncommon in most commercial poultry flocks in the United States; it is most often identified in poultry raised outdoors or those that intermingle with or are exposed to wild birds.

The AI virus that has infected birds in Asia, Europe, and Africa since the end of 2003 is highly pathogenic H5N1 avian influenza (H5N1 HPAI). This particular form of AI is deadly to most domestic poultry and some wild birds, and can spread rapidly among an entire flock. To date, H5N1 HPAI has not been found in birds in North America, including the United States, or in South America, including the Carribean. H5N1 LPAI has been detected in waterfowl and is not a serious threat.

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