The Variable Influenza A Genes
The popular media often describe influenza genes as coming from humans, birds, or pigs. However, they are not actual genes from these creatures — they are influenza virus genes from strains that evolved in those creatures. The Influenza A virus has eight genes, designated HA, NA, NP, NS, PB1, PB2, PA, and M.
Through a process called reassortment, different strains of a virus can swap genes when they infect the same individual. Reassortment means that each of the eight genes in a flu strain may have come from a different predecessor.
The popular media initially reported that the 2009 swine flu virus was "a reassortment of swine, human and avian [influenza] genes." (CNN). On the other hand, a headline from the WIRED blog "Wired Science" trumpeted in late April that "Swine Flu Genes [Are] From Pigs Only, Not Humans or Birds."