Influenza viruses belong to the Orthomyxoviridae family of viruses. Influenza viruses have an 8-segment, negative-sense ssRNA genome that encodes for at least 10 proteins, including the surface molecules haemagglutinin (H) and neuraminidase (N). Influenza A and B viruses are pathogenic for humans, with influenza A being the most common cause of seasonal epidemics. Influenza A viruses are classified according to 16 H and 9 N types. Because of the high rates of spontaneous mutations (resulting in antigenic drifts) and reassortment of influenza viruses (resulting in antigenic shifts), novel influenza strains are constantly being produced, leading to seasonal epidemics and sporadic pandemics.
Avian populations, particularly waterfowl, are the main natural reservoir for influenza viruses, and infrequently transmit influenza infections to humans. By contrast, swine populations can be infected by both human and avian influenza viruses, thereby serving as hosts for viruses of both human and avian origin. Before the 2009 pandemic, triple-reassortant swine influenza A (H1N1, H1N2, H3N2) viruses circulating among North American pigs since 1990 were causing sporadic self-limited infections in humans with exposure to pigs.  The aetiological agent of the 2009 swine influenza pandemic is a novel reassortant influenza A virus that includes segments of a triple reassortant H1N1 swine influenza virus and segments from a Eurasian H1N1 swine influenza virus.