There are three types of influenza viruses: A, B, and C ; and the latter two are primarily of a human health importance. In contrast, influenza A viruses have been isolated from a variety of birds and mammals, nevertheless the natural hosts of the virus are wild waterfowls, gulls and related birds. Other species that are infected with influenza A viruses, particularly chickens, turkeys, swine, horses, and humans, are considered aberrant hosts. Majority of influenza A viruses are fully adapted to the natural hosts in which they multiply mainly in intestines and the infection causes no symptoms. Influenza A viruses in their natural hosts are in evolutionary stasis. On the other hand, the infection in aberrant hosts usually results in rapid evolution due to selection pressure driven by the virus adaptation to a new host. Such rapid evolution can result in high virulence for the new host, and sometimes even for other species. Emergence of highly virulent influenza A viruses is of a particular concern for the poultry industry because such viruses cause up to 100% mortality in chickens and turkeys. Few influenza viruses are well adapted and established in mammalian hosts, causing primarily respiratory disease like swine influenza, equine influenza and human influenza. Swine, as an aberrant host, plays an important role in ecology and epidemiology of influenza A viruses because this species is prone to infection with viruses originating from wild birds, domestic poultry and mammalian hosts. Such a universal host can serve as a vessel for mixing of the genetic material of different viruses which can result in new influenza A viruses with unpredictable features.