Dr. Anne Schuchat , interim Deputy Director for CDC Science and Public Health , said that the American cases were found to be made up of genetic elements from four different flu viruses â `` North American swine influenza , North American avian influenza ( bird flu ) , human influenza , and swine influenza virus typically found in Asia and Europe â `` `` an unusually mongrelised mix of genetic sequences . '' Pigs have been shown to act as a potential `` mixing vessel '' in which reassortment can occur between flu viruses of several species . Of the three genera of influenza viruses that cause human flu , two also cause influenza in pigs , with influenza A being common in pigs and influenza C being rare .