[Bird populations--hatching grounds of pandemic influenza viruses?].

Aquatic wild birds constitute the natural reservoir of influenza A viruses. Under appropriate selection pressure these viruses display a remarkable genetic flexibility which is based on their high mutation rate. At least 16 subtypes of the viral hemagglutinin glycoprotein (HA) und 9 of the viral neuraminidase (NA) are distinguishable as a result of natural evolution.