Influenza viruses are clinically and economically important agents of disease in people, horses, pigs, marine mammals and poultry. Human influenza results from infection with influenza A, B or C viruses and a wide variety of domestic and free-ranging wild animal species can be infected with influenza A viruses. Aquatic birds are the natural hosts of influenza A viruses and represent a vast, global reservoir of influenza genes. Because pandemic influenza is fundamentally a zoonotic disease involving interspecies transmission of viruses from animals, scientists study the epidemiology, ecology and evolution of influenza viruses among humans, birds, and pigs. See also The Origin and Virulence of the 1918 Spanish Influenza Virus
from Kanta Subbarao, David Swayne, and Christopher W. Olsen in Influenza Virology
See also Swine flu