Reservoirs and transmission of influenza A viruses. It is postulated that wild aquatic birds are the reservoir of all influenza viruses for avian and mammalian species. There is overwhelming biological, biochemical, and epidemiological evidence () for transmission between some species, such as pigs and humans. There is strong evidence (—) for transmission between other species, such as seabirds and sea mammals, and some evidence (---) for other transmissions, such as between horses and humans.