"We want to make sure that school closure is in fact lowering the risk of spread in a community," Besser told "Good Morning America" Thursday. "The goal is not to send the children out into the community."
Vice President Joe Biden said on "GMA" Thursday that he hoped that parents would be able to make adjustments as their children's schools -- and possibly their own workplaces -- shut down to limit the threat of swine flu spread.
"Our hope is that employers will be generous in how they treat that employee," Biden said.
The school closures cap off several frenzied days of adjustments by federal and state governments to stem the spread of the virus.
And the spread appears far from over. A 30-year-old female from Kentucky with a confirmed case of swine flu is currently hospitalized in serious condition at West Georgia Medical Center in LaGrange. The woman had recently traveled to Cancun and was in Georgia for a wedding when she became ill. She has been in the hospital since April 26.
In Washington, D.C., the World Bank issued a statement Thursday morning that a staff member who had traveled to Mexico on business April 14-18 had been "preliminarily diagnosed" with swine flu. The CDC has yet to confirm the diagnosis of that individual, who is a Maryland resident, according to the statement.
The World Health Organization raised its pandemic flu alert again Wednesday to phase 5, which denotes that a pandemic is imminent. On Thursday, WHO reported that the number of lab-confirmed cases worldwide rose to 236, up from 148 on Wednesday.
In Mexico, where the flu outbreak was believed to have started, the government took more drastic action Thursday, ordering all nonessential businesses to close.
Mexican President Felipe Calderon, who has been criticized for his low profile during the crisis, addressed the nation Wednesday night and told Mexicans to stay home.
"There is no safer place to protect yourself against catching swine flu than in your house," he said.
On Thursday, Switzerland, the Netherlands and Peru became the latest countries to report swine flu cases, which have now been reported in at least 14 countries.
The first death from swine flu in the United States occurred Wednesday, when a 22-month-old baby boy in Texas died.
The worst hit state so far is New York, with 51 confirmed cases.
Mayor Michael Bloomberg originally said Monday that the cases were confined to a "single cluster," to students at St. Francis Preparatory School and their relatives. Several of the students had spent spring break in Cancun, Mexico.
Since then, Bloomberg has confirmed that it had spread to several other schools and that hundreds of children could be suffering from the virus. New York City Health Commissioner Thomas Frieden said that "many hundreds" of students and teachers at St. Francis are sick, although most of those cases appeared mild so far.
"It is here, and it is spreading," Frieden said. "We do not know whether it will continue to spread."
"I do expect more cases and expect more states to be affected," Dr. Anne Schuchat, interim deputy director for the CDC's science and public health program, told a Senate hearing Wednesday. "I think we need to be prepared that even if it starts to look a little better, it may get a little worse."