The number of confirmed U.S. cases rose to 48, most of them mild and none fatal. The government said it was shipping millions of doses of flu-fighting medicine from a federal stockpile to states along the Mexican border or where the virus has been detected.

But the American reaction to swine flu, which has killed up to 149 people in Mexico and on Monday led the World Health Organization to raise its alert level, was mostly limited to steps that hospitals, schools and mask-wearing individuals took on their own.

At the main pedestrian border crossing between El Paso and Mexico's Ciudad Juarez, a handful of people wore protective masks and officials handed out a swine flu flier provided by the Centers for Disease Control and Prevention.

But there were no extra screenings for swine flu, and it mostly looked like a typical day at the border. Suddenly faced with a new and unforeseen threat, people entering the country who said they felt unwell were questioned about their symptoms. But there were no reports of anyone refused entry.

Jorge Juarez and Miranda Carnero, both 18, crossed the border wearing bright blue masks. "It's just a precaution," said Juarez, who lives in El Paso and drew a smiley face on his mask.

Passengers from a Mexico City flight that arrived at Newark Liberty International Airport in New Jersey said they were surprised customs officials did nothing more than hand them an informational flier.

"Everyone's afraid. But when we got here, they said 'Welcome to America. You don't need that,'" said Alejandro Meneses of Fairlawn, N.J., pointing to a paper mask hanging from his neck.

The confirmed U.S. cases included 28 at a private high school in New York City, 11 in California, six in Texas, two in Kansas and one in Ohio. Only one American case has led to a hospitalization.

President Barack Obama characterized the U.S. cases as a cause for concern but not "a cause for alarm." The federal government said travel warnings for trips to Mexico would remain in place as long as swine flu is detected.

Public health experts cautioned that screenings were not foolproof. People with the flu can spread the virus to others before any symptoms show up.

"It's not a perfect solution," said Greg Gray, director of the Center for Emerging Infectious Diseases at the University of Iowa College of Public Health, who estimated the screenings would pick up 80 to 90 percent of cases.

Gray said he believed the U.S. response was appropriate given how little researchers know about the potency.

"The virus is here in North America, and it's likely to show up on every continent, I think, by the end of the week," he said. "It's hard to stop."

In other countries, precautions were far more stringent. Asian nations activated thermal scanners used during the 2003 SARS crisis to check for signs of fever among passengers arriving from North America. In Malaysia, health workers in face masks took the temperatures of passengers touching down from Los Angeles.

Australia said it would require pilots on international flights to file a report noting any flu-like symptoms among passengers before being allowed to land. And China ordered anyone with flu-like symptoms within two weeks of arrival to report to authorities.

The European Union's health commissioner urged Europeans to put off nonessential travel to part of the United States, but Dr. Richard Besser, acting head of the CDC in Atlanta, said the recommendation was unwarranted.

"At this point I would not put a travel restriction or recommendation against coming to the United States," he said.

In the U.S., protective steps were more scattered. A South Texas school district was closed, and residents of Guadalupe County, outside San Antonio, were asked to avoid public gatherings and stay home if they are ill.

Pharmacies in Manhattan reported that paper face masks were selling by the box. One pharmacy owner said he had to order more from his wholesale supplier for the first time since the SARS epidemic six years ago.

Security guards at all entrances to the University of Chicago Medical Center required anyone walking in to use a liquid disinfectant. At Rush University Medical Center, anyone seeking treatment for fever, runny nose and coughs was being tested for flu with nasal swabs.

Elsewhere, there were signs of growing unease among the public, even in places where there was no immediate known cause for alarm.

Students at a Chicago school were instructed not to shake hands with anyone, and Southern Illinois University urged students to wash their hands frequently and cover their mouths when coughing. There were no known swine flu cases in Illinois.

And in New Mexico, which also had no reported cases, health officials were so besieged by calls from concerned citizens that they set up a swine flu hot line.

In New York, all 28 confirmed cases were traced to private St. Francis Preparatory school in Queens, where pupils began lining up at the nurse's office Thursday complaining of fever, nausea, sore throats and aches. One teacher was infected.

Some of the infected students said they had recently returned from a spring break trip to Mexico. Health Commissioner Thomas Frieden said nearly all the infected students were feeling better, and none was worse.

In the subways and on the streets of the nation's largest city, it was all but impossible to find anyone wearing a mask. Mayor Michael Bloomberg said no other clusters of the virus had been detected.

"We have seen the kind of flu that does not seem to grow, and in a few days the symptoms seem to be going away," he said.