Seasonal flu comes from east and south-east Asia before spreading across the rest of the world, scientists have discovered.
The finding is important as by knowing where to focus surveillance efforts researchers may be able to extend their forecast of the flu strains most likely to cause epidemics.
This could in turn help experts decide which strains should go in the flu vaccine each year.
Researchers made the discovery after studying 13,000 samples of influenza A (H3N2) virus that were collected across six continents from 2002 to 2007.
This flu subtype is currently the major cause of flu-related illness and death.
Their analyses, described in the journal Science, revealed that strains emerge in east and south-east Asia; about six to nine months later they reach Europe and North America; and several months later still the strains arrive in South America.
"The ultimate goal of our collaboration is to increase our ability to predict the evolution of influenza viruses," said Derek Smith of the University of Cambridge, who is the corresponding author of the study.
"This study is one step along that path and in particular highlights the importance of ongoing collaborations and surveillance in east and south-east Asia, and of expanding these collaborations in the future."