They found that each year's dominant strain in Europe, the Americas or other regions was traceable to ancestors first seen in a large region of East and Southeast Asia (an area bigger than China alone). It was never directly descended from a strain seen in temperate zones the previous year. In fact, the evidence strongly suggested that flu virus goes extinct each summer in temperate climates -- there is none left to smolder and evolve.
What the researchers now believe happens is that the world is reseeded each year by new, slightly different variants. "The strains are coming out [of Asia] fully formed," Colin A. Russell, one of the researchers, said in a telephone news conference.
The route the new strains then take seems to reflect both proximity to East Asia and the amount of travel between regions. The first stops are Australia and the Pacific islands known as Oceania, which the virus reaches about three months after it arises in Asia. Three to six months later it crops up in Western Asia, Europe and then North America. The last stop is South America.
-Researchers Chart Flu's Global Journey