Bird flu is a viral infection caused by avian influenza A viruses in birds. The viruses that cause the disease in birds can mutate and spread to humans. Bird flu is named H or N depending on the protein found on the surface of the virus. Out of the three types of influenza viruses (A, B, and C), influenza A virus causes zoonotic infection. Influenza virus A is categorized into types based on surface proteins hemagglutinin (HA) and neuraminidase. Based on origin, the influenza A virus can be classified into avian influenza, swine influenza, and others types of animal influenza viruses.

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