Researchers say gene segments in the swine flu have been circulating undetected for at least a decade. This comes from a team of experts who have sequenced the genomes of more than 50 samples of the H1N1 virus.

Researchers at the Centers for Disease Control and Prevention studied samples of the flu isolated in Mexico and the United States. And they suggest that in the future, pig populations will need to be closely monitored for emerging influenza viruses. Their report was released Friday by Science, a medical and scientific journal.

First detected last month, swine flu has sickened more than 11,000 people in 41 countries and killed 85, according to the World Health Organization.