Xiang D, et al. - Given the association of avian influenza A virus (AIV) H7N9 in causing five epidemic waves of human infections in China since 2013, researchers here aimed at determining whether adaptive evolution had occurred in human-isolated H7N9 viruses. They identified that H7N9 viruses have undergone adaptive evolution to adapt to their new host environment during infection in humans. In addition, it appears that the sites where parallel evolution occurred could play roles in crossing species barriers and respond to the new selection pressures arising from their new host environments.