US 'very concerned' about swine flu outbreak US medical authorities expressed strong concern Friday about an unprecedented multi-strain swine flu outbreak that has killed at least 60 people in Mexico and infected seven people in the United States. "It's very obvious that we are very concerned. We've stood up emergency operation centers," Centers for Disease Control and Prevention (CDC) spokesman Dave Daigle told AFP. One major source of concern was that the virus included strains from different types of flu. "This is the first time that we've seen an avian strain, two swine strains and a human strain," said Daigle, adding that the virus had influenza strains from European and Asian swine, but not from North American swine. In 11 of 12 reported human cases of swine influenza (H1N1) virus infection in the United States from December 2005 to February 2009, the CDC has documented direct or indirect contact with swine.