An unidentified Texas woman has become the first known American swine flu patient to die, possibly from the disease.
However, state and local health department officials said the woman, who lived in the town of Harlingen near the Mexico border, also had recently given birth, was severely overweight, had gallstones, and had recently contracted pneumonia.
Last week, the state of Texas listed the woman as critically ill.
She was a teacher in a nearby school district, but there are currently no concerns that she might have passed the virus to students since she has not been in school for quite some time, health officials said. They also said it was unclear whether the woman had recently traveled to Mexico.
The death comes on the same day that health officials announced that schools no longer need to shut their doors to curb the spread of swine flu.
Health and Human Services Secretary Kathleen Sebelius said at a U.S. Centers for Disease Control and Prevention teleconference that the CDC guidance on school closures has changed in light of "a more mild version of the disease than what originally appeared" in the United States so far.
Dr. Richard Besser, acting director fo the CDC, said that the guidance was changed in light of the serious disruptions associated with school closures.
"For very severe pandemic, the potential benefits of school closures outweigh the risk," Besser said. "But when we get to situations approaching that of seasonal flu, then the downside of school closures outweigh the potential benefit.
"If we felt that there was a benefit there, then we would be continuing to recommend school closures."
Sebelius noted, however, that the change also puts more of a burden on parents and teachers to act responsibly if a child is ill.
"It makes it even more important that parents and teachers and others pay attention to sickness as it breaks out," Sebelius said. "This is not an indication that we know enough yet about the course that this disease will take."
Meanwhile, even though the swine flu outbreak in Mexico is waning and the country is preparing to reopen schools and businesses this week, international health officials have still not ruled out the chance of a new surge in swine flu cases.
And infectious disease experts said that it is too early to say that we are out of the woods when it comes to the disease.
Dr. Gregory Poland, professor of medicine and infectious diseases at Mayo Clinic in Rochester, Minn., said that it is "way too early to 'pull back' yet in our level of concern.
"Always expect the unexpected from influenza," Poland added.
"While there are some signs that are reassuring, less severity, decrease cases in Mexico, et cetera, I believe we are not out of the woods yet," agreed Dr. Christopher Ohl, associate professor of medicine in the section on infectious diseases at the Wake Forest University School of Medicine in Winston-Salem, N.C. "I am concerned this virus is with us for a while. It could change, become more virulent, have an increased secondary transmission rate ... in the future."
The virus, according to the World Health Organization, has now killed 27 people and sickened 1,490 in 21 countries.
And the scene is much different in other parts of the world that were hit later during the course of the outbreak.
Two Americans are among hundreds of foreigners still in quarantine in hotels on mainland China as part of the stringent measures that were put in place after a Mexican man, on a flight from Hong Kong to Shanghai, became the first confirmed case of swine flu in China, the world's most populous country.
Swine Flu May be on the Wane in North America, but Too Soon to Let Guard Down
U.S. Embassy spokeswoman Susan Stevenson told ABC News that initially a total of four Americans were put unde quarantine.
China also banned pork imports, despite continued assurances from health officials that the virus cannot be contracted by eating pork.
Mexican President Felipe Calderon, angry at reports that the Chinese had rounded up Mexicans in China even if they were not sick, and at several other countries that had banned flights to Mexico, said late Monday in a televised address that the world should "stop taking actions that only hurt Mexico and don't contribute to avoid the transmission of the disease."
A government charted plane from Mexico City arrived in China Tuesday to pick up Mexicans who were held in quarantine and wished to leave.
The United States has 380 confirmed cases in 36 states but still only one death, leading some experts to say the bug may be waning here.
In Geneva, Switzerland, today more than 150 experts will meet to compare notes on the H1N1 flu virus, including its severity and incubation period, WHO said.
Also Tuesday the United Nations will start sending enough of the anti-viral drug Tamilflu to treat 2.4 million people in 72 countries.
Some experts worry that as winter approaches in the Southern Hemisphere seasonal flu outbreaks could trigger a resurgence in the swine flu. New Zealand is the only country in the Southern Hemisphere that has a confirmed case of swine flu.
As for the CDC's analysis of the H1N1 virus, Besser said it still has not found traits condusive to pandemic caliber outbreaaks. "What we've found is that we're not seeing the factors that were associated with the 1918 pandemic, we're not seeing the factors that are, were associated with other H1N1 viruses, and that's encouraging."
CDC: More Swine Flu Cases Expected in the U.S.
Even though officials are cautiously optimistic, whether the virus will re-emerge when the typical flu season starts in the fall is still unclear.
"Every virus is new," Besser cautioned. "And what it will do is different. And so you're hitting a critical point: What will happen this spring and summer?"
He reiterated that the virus is spreading "quite easily," and that he expects to see reports of more confirmed cases in the United States.
In New York City, St. Francis Preparatory School -- last week considered the epicenter of the country's swine flu infections -- reopened Monday for the first time since the virus was first identified.
As students returned to the school, many packing bottles of hand sanitizers in their backpacks, medical experts concluded that the swine flu infection may not be any more dangerous than a normal outbreak of flu.
But the country's medical authorities aren't ready to ease restrictions, and more than 333,000 students were out of class Monday as schools across the country took no chances and shut down. Two more New York schools closed their doors Monday, including one in Syracuse and another on Long Island.
In addition, hospital emergency rooms continue to fill up with patients who have "swine flu jitters," hand sanitizers are disappearing from drug store shelves and airlines are taking unusual precautions. British Airways is handing out masks to Mexico-bound passengers. Lufthansa is putting a doctor on flights to Mexico. And Alaska Airways is eliminating pillows, hoping that will help halt the spread of germs.
The jitters persist despite efforts to tamp down fears about the potency of the H1N1 virus that has been dubbed swine flu.
And there is one worrisome pattern. While most flus strike the very young and the elderly, this virus has taken its toll on older children and young adults. Of the 30 people hospitalized in the United States with swine flu, many fit this category. And nine of the 19 deaths in Mexico attributed to swine flu were people between the ages of 21 and 39.
In the United States, the virus claimed the life of a toddler in Texas last month.
The virus spread to Colombia in the first confirmed case in South America, a fact that has worried health officials, since the flu season is about to begin on the continent. Experts are worried that typical winter flus could combine with swine flu there, creating a new strain that is more contagious or dangerous.
WHO Director-General Margaret Chan told reporters Monday that the organization would not move its alert level to Phase 6 -- meaning a pandemic has officially begun -- until "we see in another region outside North America showing very clear evidence of community-level transmission."
The Associated Press contributed to this story.