In the biggest analysis of flu strains ever, Cambridge University researchers have shown the annual flu epidemic comes from eastern and southeast Asia, a product of the connectedness of people and the patchiness of the region’s rainy seasons.

New viruses appeared in eastern and southeast Asia 6 to 9 months before they showed up anywhere else, and took another 6 to 9 months to reach Latin America.

The team found that outbreaks follow the cooler temperatures of the rainy seasons, which are staggered in time across the region because of complex geography and air movements. There is always a flu epidemic happening somewhere in east and southeast Asia, they said.

Then when the climatic conditions are right in the northern or southern temperate zones, they seed the next winter epidemic, which goes to Europe, Oceania, and North America first.