Where does the flu come from? Scientists at the University of Cambridge and the World Health Organization's Global Influenza Surveillance Network tracked the migrations of flu viruses and discovered that the most common originate in East and Southeast Asia and spread in a distinctive pattern around the world. Understanding how these viruses evolve and travel will lead to better vaccines against flu epidemics that currently infect 5 to 15 percent of the world's population each year.

Until this study was done, the global migrations of flu viruses were a mystery. To solve it, the scientists analyzed 13,000 samples of influenza A (H3N2) virus—the most common type of flu—collected worldwide by the surveillance network to determine when these strains arrived at various locations between 2002 and 2007.

Because the flu evolves so quickly, selecting the strains to include in next year's flu vaccines is a challenge. Now surveillance networks and vaccine developers can focus on Asia, which seems to be the birthplace of ever-evolving flu viruses. These viruses typically reach Europe and North America six to nine months later, and end up in South America a few months after that. Fortunately, it's usually a one-way trip.

The study will be published on April 18 in the journal Science.