Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection by any one of several types of swine influenza virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

. Swine influenza virus (SIV) or S-OIV (swine-origin influenza virus) is any strain of the influenza family of viruses

Orthomyxoviridae

The Orthomyxoviridae are a family of RNA viruses that includes five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus and Thogotovirus. A sixth has recently been described...

 that is endemic in pig

Pig

A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...

s. As of 2009, the known SIV strains include influenza C

Influenzavirus C

Influenzavirus C is a genus in the virus family Orthomyxoviridae, which includes those viruses which cause influenza. The only species in this genus is called "Influenza C virus".Influenza C viruses are known to infect humans and pigs, giving them influenza...

 and the subtypes of influenza A known as H1N1

H1N1

'Influenza A virus is a subtype of influenza A virus and was the most common cause of human influenza in 2009. 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 percent of...

, H1N2

H1N2

H1N2 is a subtype of the species Influenza A virus . It is currently pandemic in both human and pig populations.H1N1, H1N2, and H3N2 are the only known Influenza A virus subtypes currently circulating among humans....

, H2N1, H3N1, H3N2

H3N2

Influenza A virus subtype H3N2 is a subtype of viruses that cause influenza . H3N2 Viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains...

, and H2N3

H2N3

H2N3 is a subtype of the influenza A virus. Its name derives from the forms of the two kinds of proteins on the surface of its coat, hemagglutinin and neuraminidase . H2N3 viruses can infect birds and mammals....

.

Swine influenza virus is common throughout pig populations worldwide.