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Why are pigs thought to play a role in the emergence of new influenza A viruses?

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Caring for flu victims — influenza pandemic (H1N1) of 2009 Doctors at the Mexico City Navy Hospital wearing protective gear as they tend to patients complaining of H1N1 flu-like symptoms.

Pigs are considered significant in the emergence of new influenza A viruses because they can serve as "mixing vessels" for different viral strains.12

When a pig is simultaneously infected with multiple influenza A viruses—such as human, swine, and avian strains—the viruses can undergo a process called genetic reassortment. During this process, the viruses exchange genetic material, which can lead to the creation of a new, distinct influenza subtype.3124

This mechanism is particularly dangerous for humans because the resulting virus often possesses novel antigens to which the human population has little or no pre-existing immunity. This lack of immunological protection is a primary driver of pandemics, as seen in the 2009 H1N1 outbreak, which is believed to have evolved through such reassortment in pigs.4231