Pavel Novotny

The 2009 swine flu outbreak in humans is due to a new strain of influenza A virus subtype H1N1 that contains genes closely related to swine influenza. The origin of this new strain is unknown. However, the World Organization for Animal Health (OIE) reports that this strain has not been isolated in pigs. This strain can be transmitted from human to human, and causes the normal symptoms of influenza.

Pigs can become infected with human influenza, and this appears to have happened during the 1918 flu pandemic[citation needed] and the 2009 swine flu outbreak.
Of the three genera of influenza viruses that cause human flu, two also cause influenza in pigs, with Influenzavirus A being common in pigs and Influenzavirus C being rare. Influenzavirus B has not been reported in pigs. Within Influenzavirus A and Influenzavirus C, the strains found in pigs and humans are largely distinct, although due to reassortment there have been transfers of genes among strains crossing swine, avian, and human species boundaries.

Influenza C

Influenza C viruses infect both humans and pigs, but do not infect birds. Transmission between pigs and humans have occurred in the past.[9] For example, influenza C caused small outbreaks of a mild form of influenza amongst children in Japan[10] and California. Due to its limited host range and the lack of genetic diversity in influenza C, this form of influenza does not cause pandemics in humans.