Swine Flu (or Swine influenzavirus (SIV) flu) refers to a subset of Orthomyxoviridae that creates influenza in pigs and are endemic in pigs. This is not a phylogenetics based taxonomic category.

The species of Orthomyxoviridae that can cause flu in pigs are Influenza A virus and Influenza C virus but not all genotypes of these two species infect pigs.

The known subtypes of Influenza A virus that create influenza in pigs and are endemic in pigs are H1N1, H1N2, H3N1 and H3N2.

"In swine, 3 influenza A virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the world. In the United States, the classic H1N1 subtype was exclusively prevalent among swine populations before 1998; however, since late August 1998, H3N2 subtypes have been isolated from pigs. Most H3N2 virus isolates are triple reassortants, containing genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages. [...] Present vaccination strategies for SIV control and prevention in swine farms typically include the use of 1 of several bivalent SIV vaccines commercially available in the United States. Of the 97 recent H3N2 isolates examined, only 41 isolates had strong serologic cross-reactions with antiserum to 3 commercial SIV vaccines. Since the protective ability of influenza vaccines depends primarily on the closeness of the match between the vaccine virus and the epidemic virus, the presence of nonreactive H3N2 SIV variants suggests that current commercial vaccines might not effectively protect pigs from infection with a majority of H3N2 viruses."