H1N1 came from the Spanish flu, which caused a pandemic in humans in 1918 and 1919. New viruses came from the Spanish flu. These viruses became common in pigs. Every once in a while, the viruses spread from pigs to humans. This has happened during the 20th century. (However, they happened at the same time as the flu epidemics that happen every year. Because of this, it is hard to tell the difference between 'normal' influenza and swine influenza.) Infections that spread from pigs to humans are rare. Scientists only know of 12 known cases in the United States since 2005.