U.S. Government Declares Swine Flu Public Health Emergency

Napolitano also indicated that, much like Asian countries who have stepped up monitoring of visitors from Mexico, U.S. border agents have been directed to begin "passive surveillance."

Unlike the measures in some Asian countries, like Japan, this means that rather than actively monitoring body temperatures, U.S. border agents have instructions to isolate anyone who appears actively ill with swine flu.

The number of cases confirmed in the United States by the Centers for Disease Control and Prevention (CDC) has risen to 20, including eight New York City high school students. Other cases are in Ohio, California, Texas and Kansas, with ages ranging from 7 to 50-plus.

There have been no deaths in the U.S., whereas Mexico has seen at least 20, with 61 more suspected. Dr. Richard Besser, acting director of the CDC, said that compared to cases in Mexico, the U.S. disease appears milder.

Besser said it is unknown why the cases in the U.S. are so much milder. The strains have been matched genetically, so they should be affecting people in the same manner, one would expect.

Health officials are alarmed because at this point it appears that humans have no immunity to the new virus, which is a combination of avian, swine and human flu. This had led to fears of a possible global pandemic.