Swine influenza is a respiratory disease of pigs caused by type A Influenza virus. Swine flu viruses do not normally infect humans but human infections with swine flu have occurred and then spread person to person. Pigs can be infected by avian influenza and human influenza viruses as well as swine influenza viruses. When influenza viruses from different species infect pigs, the viruses can swap genes and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. Originally referred to as “swine flu,” the influenza virus responsible for the 2009 pandemic is actually a new strain of flu that consists of a mixture of genetic material from swine, avian and human influenza viruses.

http://www.cdc.gov/flu/
http://www.who.int/csr/disease/influenza/en/