Each year, most often in winter, flu viruses spread through human populations around the world. Influenza is remarkably adept at evolving anew into strains that evade human immunities and give rise to epidemics. However, no one has yet discerned where the flu lies dormant until mutations can give rise to the next virulent strain. Various theories have posited that viruses either remain hidden in local populations, retreat to the tropics, or vanish altogether. Knowing where influenza resides while dormant could help public health officials target viruses before they can cause an epidemic. A large collaboration of researchers from several countries around the world examined approximately 13,000 samples of a single major flu type. This flu had spread across six continents between 2002 and 2007, and using genetic tests and antigenetic maps from data provided by the World Health Organization, the epidemiologists were able to determine that flu viruses are constantly circulating through East and Southeast Asia before spreading out from this region to across the globe.

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