And MORE Flu: U.S. Declares Public Health Emergency As Swine Flu Spreads

(CNN) -- The United States government declared a public health emergency Sunday as the number of identified cases of swine flu in the nation rose to 20.

The declaration is part of a "standard operating procedure" that will make available additional government resources to combat the virus, Homeland Security Secretary Janet Napolitano said at the White House.

Additional cases of swine flu are expected to be reported in the coming days, added Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention.

No one has died in the U.S. from swine flu, officials said Sunday.

In New York City, Mayor Michael Bloomberg said eight students at St. Francis Prepatory School in Queens have tested positive for swine flu. More than 100 students at the school were absent with flu-like symptoms last week, he said.

State public health officials in Ohio confirmed one case of swine flu on Sunday. There have been seven confirmed cases in California, two in Kansas, and two in Texas, Besser said.

The World Health Organization advised all countries to be on the lookout for "unusual" outbreaks of flu, after an emergency meeting Saturday as the seriousness of the outbreak became clear.