Influenza A (H1N1) virus is a subtype of influenzavirus A and the most common cause of influenza in humans. Some strains of H1N1 are endemic in humans and cause a small fraction of all influenza-like illness and a small fraction of all seasonal influenza. H1N1 strains caused a few per cent all human flu infections in 2004-2005. Other strains of H1N1 are endemic in pigs and in birds. In June 2009, World Health Organization declared that flu due to a new strain of swine-origin H1N1 was responsible for the 2009 flu pandemic. This strain is often called "swine flu" by the public media.