Contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico, federal agriculture officials now believe that it most likely emerged in pigs in Asia, but then traveled to North America in a human. But they emphasized that there was no way to prove their theory and only sketchy data underpinning it. There is no evidence that this new virus, which combines Eurasian and North American genes, has ever circulated in North American pigs, while there is tantalizing evidence that a closely related