In Washington, President Barack Obama told Americans the government was "taking the utmost precautions and preparations" to stop the virus and urged people to wash their hands, cover their coughs and stay home when they feel sick.
The Mexican government also ordered all but the most essential government services shut down from Friday through Tuesday, meaning only functions like transport, supermarkets and hospitals will stay open. School had already been canceled until Wednesday.
The steps were aimed at stopping further spread of the virus, blamed for 170 deaths in Mexico and one in the United States.
"There is no safer place to protect yourself against catching swine flu than in your house," Mexican President Felipe Calderon said in a televised address Wednesday night. He praised the "heroic work" of doctors and nurses and brushed aside criticism that his government's response was slow.
In the U.S., where swine flu has been confirmed in 11 states, both Vice President Joe Biden and the acting head of the Centers for Disease Control and Prevention said in televised interviews Thursday there would be no practical benefit to closing the U.S.-Mexican border. Biden said on CBS that it would be a "monumental undertaking" with far-reaching consequences.
Biden also said on NBC's "Today" show that he is advising his own family to stay off commercial airlines and even subways because of swine flu. If one person sneezes on a confined aircraft, he said, "it goes all the way through the aircraft." His precautions go beyond official advice from the U.S. government.
The WHO on Wednesday raised its alert level to Phase 5, the second-highest, indicating a pandemic may be imminent.
"It really is all of humanity that is under threat during a pandemic," WHO Director General Margaret Chan said in Geneva. "We do not have all the answers right now, but we will get them."
Switzerland and the Netherlands became the latest countries to report swine flu infections. In addition to Mexico and the U.S., Canada, New Zealand, Britain, Germany, Spain, Israel and Austria have confirmed cases.
The Swiss government said a 19-year-old student with swine flu was mistakenly released from the hospital and then hastily readmitted. The Dutch said a 3-year-old child who recently returned from Mexico had contracted swine flu and was being treated and recovering well.
European Union health ministers planned emergency talks in Luxembourg to coordinate national efforts in preventing the spread of swine flu in Europe.
The United States confirmed its first swine flu death on Wednesday, a Mexican toddler who visited Texas with his family and died Monday night in Houston. Thirty-nine Marines were confined to their base in California after one came down with the virus, a mix of pig, bird and human genes to which people have limited natural immunity.
The outbreak appeared to already be stabilizing in Mexico, the epicenter. Confirmed swine flu cases doubled Wednesday to 99, but new deaths finally seemed to be leveling off after an aggressive public health campaign was launched when the epidemic was declared April 23.
Although 17 new suspected deaths were reported, only one additional confirmed death was announced Wednesday night, for a total of eight countrywide. The virus is believed to have sickened as many as 2,955 people across the country, though hospital records suggest the outbreak may have peaked here last week.
Calderon said authorities would use the partial shutdown to weigh whether to extend the emergency measures, or "if it is possible to phase out some" restrictions.
The WHO said the global threat was serious enough to ramp up efforts to produce a vaccine against the virus. It declared a Phase 5 outbreak for the first time.In the U.S., the CDC and state officials have confirmed cases in New York, Texas, California, Maine, Kansas, Massachusetts, Indiana, Ohio, Arizona, Nevada and Michigan. Eight states closed schools Wednesday, affecting 130,000 students in Texas alone.
Obama said his administration has made sure that needed medical supplies are on hand and he praised the Bush administration for stockpiling 50 million doses of antiviral medications.
"The key now is to just make sure we are maintaining great vigilance, that everybody responds appropriately when cases do come up. And individual families start taking very sensible precautions that can make a huge difference," he said.
Ecuador, Cuba and Argentina have all banned travel to or from Mexico, and Peru has banned inbound flights. The Panama Canal Authority ordered pilots and other employees who board ships passing through the waterway to use surgical masks and gloves.
The U.S., the European Union and other countries have discouraged nonessential travel to Mexico. Some countries have urged their citizens to avoid the United States and Canada as well. Health officials said such bans would do little to stop the virus.
In France, President Nicolas Sarkozy met with Cabinet ministers to discuss swine flu, and the health minister said France would ask the European Union to suspend flights to Mexico.
Medical detectives have not pinpointed where the outbreak began. Scientists believe that somewhere in the world, months or even a year ago, a pig virus jumped to a human and mutated, and has been spreading between humans ever since.
China has gone on a rhetorical offensive to squash any suggestion it's the source of the swine flu after some Mexican officials were quoted in media reports in the past week saying the virus came from Asia and the governor of Mexico's Veracruz state was quoted as saying the virus specifically came from China.
One of the deaths in Mexico directly attributed to swine flu was that of a Bangladeshi immigrant, said Mexico's chief epidemiologist, Miguel Angel Lezana.
Lezana said the unnamed Bangladeshi had lived in Mexico for six months and was recently visited by a brother who arrived from Bangladesh or Pakistan and was reportedly ill. The brother has left Mexico and his whereabouts are unknown, Lezana said. He suggested the brother could have brought the virus from Pakistan or Bangladesh.
By March 9, the first symptoms were showing up in the Mexican state of Veracruz, where pig farming is a key industry in mountain hamlets and where small clinics provide the only health care.
The earliest confirmed case was there: a 5-year-old boy who was one of hundreds of people in the town of La Gloria whose flu symptoms left them struggling to breathe.
Days later, a door-to-door tax inspector was hospitalized with acute respiratory problems in the neighboring state of Oaxaca, infecting 16 hospital workers before she became Mexico's first confirmed death.
Neighbors of the inspector, Maria Adela Gutierrez, said Wednesday that she fell ill after pairing up with a temporary worker from Veracruz who seemed to have a very bad cold. Other people from La Gloria kept going to jobs in Mexico City despite their illnesses, and could have infected people in the capital.
Jose Cordova, the Mexican health secretary, said getting proper treatment within 48 hours of falling ill "is fundamental for getting the best results" and suggested the virus can be beaten if caught quickly and treated properly.
But it was neither caught quickly nor treated properly in the early days in Mexico, which lacked the capacity to identify the virus, and whose health care system has become the target of widespread anger and distrust. In case after case, patients have complained of being misdiagnosed, turned away by doctors and denied access to drugs.
Swine flu has symptoms nearly identical to regular flu - fever, cough and sore throat - and spreads like regular flu, through tiny particles in the air, when people cough or sneeze. People with flu symptoms are advised to stay at home, wash their hands and cover their sneezes.