text: Actually H5N1 is an older strain.  The strains have been tracked since the ability to recognize the specific types.  \n"Influenza A subtypes are defined by the expression of two key surface proteins: hemagglutinin (H) and neuraminidase (N). Sixteen hemagglutinins are known to exist, each of which can be “paired” with one of nine neuraminidase proteins. These influenza A subtypes are all maintained in aquatic birds, which serve as a continual source of new viruses. "\nOlder strains have only recently been tracked as a specific HN subtype.  The H5N1 first appeared in the literature in 1985.