Influenza H1N1 virus (swine flu) was a new flu virus strain that caused a worldwide pandemic in humans from June 2009 to August 2010. Earlier forms of the H1N1 virus were found in pigs. Over time, the virus changed (mutated) and infected humans. H1N1 was a new virus in humans in 2009, and it spread quickly around the world. Most of the people who got the H1N1 flu were children and young adults. Infection with the 2009 H1N1 virus causes a broad spectrum of clinical syndromes, ranging from afebrile upper respiratory illness to fulminant viral pneumonia.