Humans, wild and domestic birds, horses, pigs, and marine mammals can be infected by influenza.
Theses are reservoirs of influenza viruses, which appear to be species specific.
People or animals that are recovering from infection act as reservoir hosts between epidemics. Antigenic shift, or mutation, is probably required before an animal virus becomes able to infect humans. Sporadic human infections with swine and avian strains have been reported. Human cases of influenza have occurred from contamination by aerosols from infected ferrets and vice versa. Most cases of influenza in humans result from direct contact with an infected person.