Avian flu is normally only transmitted among poultry and rarely from poultry to people, but it can't move among mammals, at least not yet. To study how the virus might evolve this ability, Kawaoka and Ron Fouchier of the Erasmus Medical Center in the Netherlands induced several changes to its genetic makeup. In Kawaoka's experiment, researchers combined elements of the H5N1 flu with a human flu, the 2009 H1N1 "swine flu" that caused an epidemic. They figured out which mutations would help the bird virus shift to a variant that would thrive in humans. Read more about his experiment here. In Fouchier's experiment, researchers changed three amino acids to make the virus adapt to mammalian cells, and then infected several groups of ferrets. In both cases, the ferrets got sick. You can learn more about his methods here.