"A unique epidemiological feature of the 1918 influenza virus, related to its origin, was infection of both humans and swine. Influenza was first recognized as a clinical entity in swine in the United States in autumn 1918, concurrent with the spread of the pandemic in humans, having apparently been transmitted from humans to pigs. This host switch split the virus off into two independent viral lineages, one human and the other porcine. After 1918, the epizootic disease became widespread among herds of swine in the U.S. midwest. Epizootic viruses appeared annually thereafter, leading to Shope’s 1930 isolation of the first influenza virus, A/swine/Iowa/30, 3 years before the first human isolation of a descendant of the parent 1918 virus, A/WS/33. The two 1918 viral H1N1 lineages, one human and the other porcine, evolved and antigenically drifted at different rates until 2009. In the 2009 pandemic, the human-adapted H1N1 descendant was replaced by a different H1N1 virus that was also a 1918 viral descendant, ironically one that had been circulating enzootically in pigs. The original 1918 classical swine lineage still circulates enzootically today."