A virus engages a host cell in a manner somewhat similar to the way a key fits a lock. The host cell's receptor sites must be stereochemically compatible with antigens expressed on the surface of the virus's outer protein jacket. If this compatibility is totally lacking, the virus cannot infect a host. That's one reason why a man cannot be infected by a tobacco virus and a tobacco plant can't develop measles. Hemagglutinin is a protein believed to facilitate binding of influenza virus particles onto endothelial cells inside the respiratory tree. The linked article from recombinomics.com asserts the emergency in Turkey of a genetic mutation so altering hemagglutinin synthesis as to make the virus more efficiently transmissible from person-to-person. "Efficient Transmission of H5N1 to Humans in Turkey" 15 January 2006 http://www.recombinomics.com/News/01150601/H5N1_Turkey_Efficient.html Uncertainty as to the number of human cases leaves a lot of room for conjecture as to just how big a menace we are facing.