In 1998, the U.S. Department of Agriculture (USDA) began surveillance of swine for influenza virus in the U.S. An influenza virus that was a hybrid of human, bird and swine flu viruses was detected in pigs. This virus became the dominant flu virus in U.S. pigs by 1999.

Since 1998, IAV-S infections in the U.S. have evolved from a seasonal disease caused by a single, relatively stable H1N1 genotype to an endemic, year-round respiratory disease caused by multiple genetically unstable IAV-S subtypes (H1N1, H1N2, and H3N2).