Influenza A viruses have a segmented, negative-stranded RNA genome. These viruses are classified according to the antigenic properties of the two glycoproteins, expressed on the surface of the virus particles, the hemagglutinin (HA or H) and the neuraminidase (NA or N). To date, 17 H and 10?N have been described and 116 HxNy combinations or subtypes reported. Except for the H17N10 subtype recently identified in bats, all identified subtypes have been identified in wild aquatic birds. These birds are considered to be the natural reservoir of influenza A viruses, from which some subtypes can be transmitted to other bird and mammal species, including humans. Interspecies transmissions seem to occur regularly, and can occasionally lead to the adaptation and stable establishment of a new viral lineage in a given species. This review recalls the genetic diversity of avian, swine and human influenza viruses and focuses on lesser-known influenza A viruses, identified in horses, dogs and very recently in bats. It discusses the genetic mixing that may result from interspecies transmission, and the associated risks of epizootics, zoonosis and pandemics.