View larger image | Biological barriers — several of which are related to the human upper respiratory tract — stand in the way of a bird flu strain becoming highly transmissible among humans, scientists say. Late last year, though, two teams reported making a lab-made virus capable of overcoming some of these obstacles. Credit: Respiratory system: Matthew Cole/Shutterstock, adapted by T. Dubé; virus: T. Dubé; ribbon structure: H.-I. Yen and J.S.M. Peiris/Nature 2012