(a) Gene reassortment Pigs become infected with duck virus and human virus at the same time, eg on a chinese commune where all 3 species live close together. Some virus reassorts its RNA segments in the pig respiratory epithelial cells to produce a new virus with a duck H gene for attachment and a 7 human genes for virus growth. This new virus will infect vaccinated humans because it has a brand new subtype of H.

(b) Change of species specificity. A 1997 virus which killed at least 8 Hong Kong children was related in all 8 gene segments to an H5 chicken fowl plague virus. This HK virus could have originated from migratory ducks, spread to chickens, undergone mutations to become more virulent in chickens and thereby become infectious to man. For this reason a million chickens were slaughtered in HK in 1998 although the human cases had already stopped.

Research : To trace the global cross-species transfer of influenza viruses the MRC at Mill Hill have been awarded a major Veterinary Epidemiology grant to see which animals in which countries have antibodies to H5.