Scientists conducting the study, funded in part by the National Institute of Allergy and Infectious Diseases, said the fact pigs can be infected by swine viruses, bird viruses and human flu viruses means they act as virtual virus "mixing bowls."
"The worry is if a pig were to become simultaneously infected with both a human and an avian influenza virus, genes from these viruses could reassemble into a new virus that could be transmitted to, and cause disease in, people," said NIAID Director Anthony Fauci.
The U.S. swine industry has shifted during the last 60 years from small herds on primarily family farms to large herds maintained in large, confined facilities. The crowded conditions make swine flu infections among pigs a year-round occurrence, rather than the seasonal event they once were.
The study appears this week at the Web site of the journal Clinical Infectious Diseases.