Influenza A viruses are classified into subtypes based on their surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). All virus subtypes circulate among waterfowl. Some are detected in various mammals such as horses, cats, seals, and pigs. Cases of transmission of H1N1 swine viruses to humans have been reported on multiple occasions. In the United States, from 2005 to 2009, so-called "triple-reassortant" swine viruses were responsible for sporadic human cases of respiratory infection. The new A(H1N1)v virus circulating today shares its genomic segments with these viruses and another Eurasian swine strain. In France, the A(H1N1)v virus is detected using an RT-PCR method specific to the H1v gene, developed by the National Reference Centers (CNR) for influenza viruses. Genetic and antigenic analysis reveals no significant differences between viruses isolated in France and elsewhere in the world. Currently, all isolated viruses are sensitive to neuraminidase inhibitors such as oseltamivir and zanamivir. Although the severity appears moderate at present, increased virulence could emerge at any time, necessitating close monitoring of the evolution of this new virus. (R.A.)