It would be interesting to dissect the factors that govern waypoint sources and sinks. For example, air travel and other transportation may play a major role in the dispersal of virus worldwide [8, 19, 34, 35] . Many important hubs of the global flu network, including USA, Australia, Hong Kong, and China, have several of the world's busiest airports [36] . Understanding the reasons for these seeding patterns may offer other strategies for arresting the movement of flu.