Like the other flu viruses, swine flu viruses are subject to continuous change. Pigs may develop also avian flu or any of the human flus, in addition to swine flu. Such cases occur when new viruses come into existence due to gene modifications. 4 basic subtypes of type A swine flu virus have been isolated: H3N2, H3N1, H1N2, and H1N1, the most isolated subtype being H1N1.
