Media reports have suggested that swine flu originated on Mexican factory farms, but U.S. agriculture officials now believe it probably first emerged in pigs in Asia, and travelled to North America after infecting a human, the New York Times reports. However, the theory has so far been impossible to prove. “The most likely scenario is that it came over in the mammalian species that moves most freely around the world,” said Dr. Amy L. Vincent, a swine flu specialist at the Agriculture Department’s laboratory in Ames, Iowa. However, that person will probably never be found, as people stop carrying the virus when their health improves. But a sample from a Hong Kong pig, taken in 2004, was recently found to have a virus nearly identical to the new flu. Scientists say there’s been far too little surveillance of swine flu around the world. “Something could have been going on there for a long time and we wouldn’t know,” said Dr. Michael W. Shaw, a scientist in the flu division of the Centers for Disease Control and Prevention.