While H1N1 swine viruses have been known to circulate among pig populations since at least 1930, H3N2 influenza viruses did not begin circulating among US pigs until 1998.

• H1N1猪流感病毒早在1930年就被发现在猪群中传播，而H3N2流感病毒则直到1998年才发现在美国传播开来。