All swine influenza strains are influenza A viruses, which are classified according to subtypes of the HA (haemagglutinin) and N (neuraminidase) surface antigens. The 2009 swine influenza outbreak was caused by a novel H1N1 influenza A virus.
For epidemiological and reporting purposes, the WHO use the following case definitions of influenza illnesses.
Influenza-like illness: fever and sore throat and/or cough in absence of another known cause.
Probable case: influenza-like illness with positive test for influenza A but negative for H1 and H3 by real-time reverse transcriptase-PCR (rRT-PCR).
Confirmed case: influenza-like illness with confirmed H1N1 influenza infection based on rRT-PCR or viral culture.