Federal officials closely watched developments as at least two dozen new cases of swine flu were reported.
President Barack Obama, whose administration has taken a high profile to offer reassurances and advice about the threat, expressed hopes the swine flu will run its course "like ordinary flus." Health officials suggested the virus now appears less ominous than it did at first.
"I'm optimistic that we're going to be able to manage this effectively," Mr. Obama said Friday. But he said the federal government is preparing as if the worst is still to come so that it won't be caught flat-footed.
"This is a new strain of the flu virus, and because we haven't developed an immunity to it, it has more potential to cause us harm," the president said in his radio and Internet address Saturday. "Unlike the various strains of animal flu that have emerged in the past, it's a flu that is spreading from human to human. This creates the potential for a pandemic, which is why we are acting quickly and aggressively."
The government issued new guidance for schools with confirmed cases, saying they should close for at least 14 days because children can be contagious for seven to 10 days from when they get sick. That means parents can expect to have children at home for longer than previously thought.
The Education Department said that more than 430 schools had closed, affecting about 245,000 children in 18 states. That was about 100 more schools reported closed than reported on Thursday.
The latest developments in the flu scare - more intense in neighboring Mexico than in the United States - came as the Centers for Disease Control and Prevention reported the virus had been confirmed in more U.S. states.
Confirmed cases had risen from 109 on Thursday to 161 Friday, the CDC said, with the flu reported in about double the number of states as the 11 reported Thursday. The U.S. death toll remained at one - the Mexican toddler who visited Texas with his family and died there.
Mexico has confirmed more than 300 swine flu cases and has 16 confirmed deaths, although reports have indicated that roughly 120 may have died from it.
Worldwide, the total confirmed cases numbered more than 650, although officials believe there are many more.
The CDC said that the virus was continuing to spread, though no faster than the rate of the regular winter flu.
"We think the cases do continue to occur," said CDC's Dr. Anne Schuchat. But CDC also said the new swine flu virus lacks genes that made the 1918 pandemic strain so deadly.
While emphasizing at a news conference that the closures to date represent a tiny fraction of the almost 100,000 schools in the country, Duncan instructed teachers, parents and students to be prepared if their school does close.
To teachers, Duncan said: "Think about reworking upcoming lesson plans so students can do their schoolwork at home if necessary."
To parents: "Learn about what they're learning at school. Keep them on task."
And to students: "Don't fall behind your peers at other schools that are still in session. Keep working hard."
Major U.S. airlines announced plans to curtail flights into Mexico.
Many travelers have become increasingly concerned about going to Mexico, though authorities there said new cases were leveling off.
U.S. travelers have been advised to avoid nonessential travel to Mexico. Continental Airlines Inc., the biggest U.S. carrier to Mexico, said Friday it would halve the number of seats it sells to fly there. Delta Air Lines Inc. and UAL Corp.'s United Airlines also announced plans for reduced flights to Mexico, while smaller carriers were following suit.
Though most U.S. cases have been relatively mild and have not required a doctor's visit, U.S. precautions include shipping millions of doses of anti-flu drugs to states in case they're needed, replenishing the U.S. strategic stockpile with millions more treatment courses, and shipping 400,000 treatment courses to Mexico.
2009 H1N1 Flu Outbreak Map:
This is a map depicting confirmed and suspected cases of the 2009 H1N1 outbreak, with contributors from all over the world, from a variety of backgrounds including health, journalism, technology.