The 2014-15 outbreak of avian flu was likely driven by long-distant migrant birds, a new study reveals. The finding could help inform future efforts to thwart outbreaks. Beginning in 2014, a highly infectious strain of avian influenza, H5N8, caused disease outbreaks in domesticated chickens, first in South Korea, before spreading to other countries in Asia, Europe, and North America. However, it is not clear by which routes the virus spread so rapidly around the world. To gain more insights, a group of researchers from the Global Consortium for H5N8 and Related Influenza Viruses analyzed the variation in samples of the virus taken from different locations and times during the outbreak. As well, they analyzed the migration patterns of wild birds infected with the virus, reports of outbreaks, and poultry-trade records from countries where H5N8 was reported. Their results suggest that the virus spread along two main long-distance migration routes in wild birds: one from the east Asia coast/Korean peninsula, north to the Arctic coast of the Eurasian continent, then west to Europe; and the second route from the Korean peninsula, then east across the Bering Strait, and south along the northwest coast of North America. The authors consider direct contact with infected wild birds or indirect contact with materials contaminated with wild-bird feces to be the most likely route of transmission. A Perspective by Colin Russell discusses these findings in greater detail.