Researchers have long assumed that most flu strains originated in China and Southeast Asia, but according to a team of U.S. researchers, not all strains of flu that circulate through North America die off at the end of influenza season, suggesting the U.S. could incubate some strains. They appear to then migrate to warmer climates, Reuters reports, moving on to South America or beyond, which may have happened with the H1N1 flu pandemic. “We found that although China and Southeast Asia play the largest role in the influenza A migration network, temperate regions—particularly the USA—also make important contributions,” said the University of Michigan’s Trevor Bedford, whose study is published in the Public Library of Science journal PLoS Pathogens. This could help change strategies to fight flu: for instance, using antiviral drugs too aggressively could promote drug resistance if strains don’t die out in the U.S.