When even John McCain calls to close the Mexican border over the potential swine flu pandemic, you know it's serious. Except, he left himself an out indicating that he might just be blowing smoke (link):

"At the same time, many questions still remain, including the question of why the cases of the H1N1 influenza in Mexico appear to be more severe and deadly than the cases seen in the U.S. I continue to believe that all available options to end this crisis must remain under consideration, including closing the border if it would prevent further transmission of this deadly virus."

Of course, Barack Obama, Janet Napolitano, and the World Health Organization say that it wouldn't help, even at the same time as Napolitano seems to contradict herself.