The new influenza A H1N1 virus: balancing on the interface of humans and animals.

In the spring of 2009, a new human influenza A H1N1 virus emerged in Mexico and the United States. The strain was referred to as "swine flu" as it has strong similarities with current circulating swine influenza viruses, although the first outbreak on a swine farm was recorded more than 2 mo following the first human reports. This new strain, designated as pandemic (H1N1) 2009, has shown the ability to spread amongst the human population and can be found on all continents. The way influenza viruses and specifically this influenza A pandemic (H1N1) 2009 virus evolve is described in this manuscript.