From NPR Blog: (editing is mine)
By Richard Knox
Dr. Anthony Fauci, director of the National Institute of Allergies and Infectious Diseases, says the MIT reports are "important evidence" of how the flu viruses are evolving. "However, that is not the entire story, at all, of how viruses evolve in a way to have sustained transmission," he says.
Fauci says what makes animal flu viruses highly contagious among people is complex and poorly understand. "In fact," he says, "I'm not even sure what the extent of the complexity is."