Influenza A virus has one of sixteen possible hemagglutinin (HA) surface proteins and one of nine possible neuraminidase (NA) surface proteins. In early 2009, a novel H1N1 swine-origin influenza (S-OIV) A virus was identified in specimens obtained from patients in Mexico and the United States. The genetic make-up of this swine flu virus is unlike any other: it is an H1N1 strain that combines a triple assortment first identified in 1998 including human, swine, and avian influenza with two new pig H3N2 virus genes from Eurasia, themselves of recent human origin.