The global spread of H5N1 highly pathogenic avian influenza viruses and their ability to infect not only birds but humans emphasises that human and animal health are unavoidably linked. At present avian influenza remains an animal disease problem under urgent need for control but control in birds will also reduce the potential for a human influenza pandemic. Our knowledge of the behaviour of avian influenza viruses in domestic fowl and wild birds is limited. This proposal poses some fundamental questions that address how the easily the virus can infect chickens, turkeys and ducks; how much, and for how long, virus is shed following infection in each species; and how avian influenza virus infection is controlled by the immune response of birds. Fundamental studies of this type will be critical to the design and implementation of control measures in the short term and the long term.