There is consensus that the migratory waterfowl of the world (predominantly wild ducks) serve as the natural reservoirs of all influenza A viruses, which cause asymptomatic infection in these birds. Influenza viruses have probably co-evolved with ducks over millennia, establishing an equilibrium between hosts and parasites so that neither suffers a significant loss of biological fitness; the evidence being minimal signs of disease in the hosts and the annual isolation of common subtypes. The unanswered question is whether these migratory bird species are the reservoirs of the currently circulating H5N1 HPAI viruses. Until the emergence of the Asian H5N1 HPAI viruses, the available data indicated that each new outbreak of HP H5 or H7 virus died out or was stamped out and that subsequent HP strains emerged from the low-pathogenic H5 and H7 virus reservoir.