Swine influenza virus (referred
to as SIV) refers to influenza cases that are caused by
Orthomyxoviruses endemic to pig populations. SIV strains isolated to
date have been classified either as Influenzavirus C or one of the
various subtypes of the genus Influenzavirus A.

Swine flu
infects people every year and is found typically in people who have
been in contact with pigs, although there have been cases of
person-to-person transmission. Symptoms include fever, disorientation,
stiffness of the joints, vomiting, and loss of consciousness ending in
death. Swine influenza is known to be caused by influenza A subtypes
H1N1, H1N2, H3N1, H3N2, and H2N3.

In swine, three influenza A
virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the
world. In the United States, the H1N1 subtype was exclusively prevalent
among swine populations before 1998; however, since late August 1998,
H3N2 subtypes have been isolated from pigs. As of 2004, H3N2 virus
isolates in US swine and turkey stocks were triple reassortants,
containing genes from human (HA, NA, and PB1), swine (NS, NP, and M),
and avian (PB2 and PA) lineages.