Swine influenza viruses may circulate among swine throughout the year, but most outbreaks occur during the latefall and winter months similar to outbreaks in humans. At this time, there are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. However, most of the recently isolated influenza viruses from pigs have been H1N1 viruses.