Technical Abstract: The ecology of influenza A viruses is very complicated involving multiple host species and viral genes. Avian species have variable susceptibility to influenza A viruses and they are the reservoir for this group of pathogens. Occasionally, influenza A viruses are transmitted to mammals from avian species, which can lead to the development of human pandemic strains by direct or indirect transmission to man. Because swine are also susceptible to infection with avian and human influenza viruses, genetic reassortment between these viruses and/or swine influenza viruses can occur. The potential to generate novel flu viruses has led swine to be called the "mixing vessel". The mixing vessel theory is one mechanism by which unique viruses can be transmitted from an avian reservoir to man. Although swine can generate novel flu viruses capable of infecting man, at present it is difficult to predict which virus, if any, will induce a human pandemic. Clearly, the ecology of influenza A viruses is dynamic and can impact the health of man and his companion animals as well as the health of livestock and poultry for production of valuable protein commodities. Based on history, influenza is, and will continue to be, a serious threat to the wellbeing of mankind.