A sign at the Starbucks cafe in the Polanco neighborhood in Mexico City, informs April 28, 2009 that the house will be temporarily closed. 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 60 people in Mexico is a "serious situation" with a "pandemic potential", the head of the World Health Organization said Saturday.