Pigs are seen at a swine farm in Rio Negro, outkirts of Medellin, Colombia on April 28,2009. An outbreak of deadly swine flu in Mexico and the United States has raised the specter of a new virus against which much of humanity would have little or no immunity. The outbreak of the new multi-strain swine flu virus transmitted from human to human that has killed up to 149 people in Mexico is a 'serious situation' with a 'pandemic potential', the head of the World Health Organization said Saturday. By Raul Arboleda/AFP/Getty Images.