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In 1878 a disease capable of causing extremely high mortality among infected birds was first identified as fowl plague. Only type A influenza viruses are known to cause natural infections of birds, but viruses of all 16 HA and all 9 NA influenza A subtypes in most of the possible combinations have been isolated from avian species.

Since 2003, the highly pathogenic avian influenza (HPAI) A(H5N1) viruses have spread throughout southeast Asia and to several other regions of the globe. Two subtypes of avian influenza viruses are known to exist in highly pathogenic form: H5 and H7. Both subtypes are associated with widespread death in poultry and substantial economic loss to farmers. Additionally, A(H5N1) has been reported to infect nearly 400 people with a mortality rate of about 60%. The high pathogenicity of H5N1 influenza viruses and their capacity for transmission from birds to human beings has raised worldwide concern about an impending human influenza pandemic similar to the notorious H1N1 Spanish influenza of 1918.

Evidence that HPAI strains arise from low pathogenic avian influenza (LPAI) strains has led the World Organization for Animal Health (OIE) to classify outbreaks/infections caused by all H5 or H7 strains as notifiable.