The World Health Organization has raised its pandemic swine flu alert level to phase 5 -- its second-highest level.
The move by the agency, which is the public health arm of the United Nations, is "a strong signal that a pandemic is imminent," according to published guidelines.
"The change to a higher phase of alert is a signal to governments [and industry] that certain actions now should be undertaken with increased urgency and at an accelerated pace," said Dr. Margaret Chan, WHO's director-general, at a press conference Wednesday.
"The biggest question is this -- how severe will the pandemic be, especially now at the start," Chan said. "It really is all of humanity that is under threat during a pandemic."
Practically speaking, the shift is crucial to mobilize pharmaceutical companies and governments to start manufacturing adequate antiviral drugs and expedite the creation of a vaccine against swine flu.
It also places the world a single step away from an official global pandemic.
President Barack Obama pledged "great vigilance" in confronting the swine flu outbreak Wednesday night as it spread coast to coast across the U.S.
Despite calls from many U.S. lawmakers for tightening controls over the Mexico-US border, Obama ruled out that option, even though the swine flu outbreak has been at its most virulent and may have begun there.
"We have been preparing all along as if this is going to be stage 6," Secretary of Homeland Security Janet Napolitano said at a press conference this afternoon before the WHO announcement. "Our efforts have been to stay ahead of whatever number WHO assigns."
Health and Human Services Secretary Kathleen Sebelius said at the same conference that the change in designation should not inspire undue worry.
"Panicking is probably not very helpful," she said. "What's helpful is turning this concern and worry into action steps."
In the United States, the swine flu virus has spread to 11 states and infected 94 people, according to the U.S. Centers for Disease Control and Prevention.
Appoximately 160,400 children across fourteen states will stay home from school on Thursday.
Five more states -- Arizona, Michigan, Mass., Nevada and Maine – each now have at least one confirmed case.
The latest state to report infections is Maine, with three new cases, according to Gov. John Baldacci, who says all those stricken are at home recovering .
In addition to the Maine, Richard Besser, acting director of the CDC, said there were now 51 cases in New York; 16 cases in Texas; 14 cases in California; two cases each in Massachusetts, Michigan and Kansas; and single cases in Arizona, Nevada, Indiana and Ohio.
The CDC has not officially added the Maine infections to their totals, which stand at 91 confirmed cases.
The announcements came on the same day that federal health officials reported the swine flu death of a 22-month-old baby boy in Texas -- the first known U.S. fatality from the outbreak.
The baby, who died at Texas Children's Hospital in Houston Monday, was from Mexico and was in the United States to visit family in nearby Brownsville, Texas, officials said. The death is the first linked to swine flu to occur outside of Mexico.
"This is obviously a serious situation," said President Obama of the developments today. He further urged local health departments to watch for new cases -- and for schools to "strongly consider" closing their doors if the virus is suspected or detected among students or staff.
The first U.S. swine flu fatality and 16 confirmed swine flu cases in Texas have sent shockwaves through local communities. Schools closed their doors as a precaution, keeping nearly 53,000 students in Texas home today.
"This is nothing to panic over, but there are precautions we need to follow to prevent the spread of this illness," Cleburne Mayor Ted Reynolds said.
Every high school sporting event and most extracurricular activities throughout the state have been postponed until the middle of May as a result of the outbreak.
"I'd rather be inconvenienced than have these kids sick and contaminating everybody," said Jane Stone Pritzkaw, parent from New Branufels, Texas, where there is one confirmed case in the district.
Schools across the country -- from Connecticut to San Francisco -- followed suit. In Chicago, officials closed a school on the city's North Side after a "probable" swine flu infection was reported Tuesday. Blood samples are being sent to the CDC for analysis. If confirmed, Illinois would become the 12th state to report an outbreak.
Germany and Austria today became the latest countries to confirm swine flu infections.
The Egyptian government has begun slaughtering all 300,000 pigs in the country, according to local reports. No swine flu cases have yet been confirmed on the country, though its neighbor Israel has reported two. And EU foreign relations commissioner Benita Ferrero-Waldner told Reuters during a Gulf Arab foreign ministers' meeting in Muscat on Wednesday that the bloc was considering halting all travel to Mexico and disinfecting all airports due to the outbreak.
Governments around the world are struggling to contain the disease, but no one is ready yet to call the outbreak a full fledged pandemic.
"It's a very serious possibility, but it is still too early to say that this is inevitable," the World Health Organization flu chief, Dr. Keiji Fukuda, told reporters today.
Nine countries have officially reported 148 cases of swine influenza, According to WHO.
The death comes as public fears over swine flu continue to grow. Before this fatality, health officials confirmed Tuesday that five patients in the United States have been hospitalized with swine flu; all have recovered.
And last night, a flight from Baltimore to Mexico was, for a time, suspected of carrying a passenger with swine flu. The airport had prepared to quarantine the plane before authorities said the concern was unwarranted.
Both the Obama administration and the Centers for Disease Control and Prevention have been warning for days that American deaths should be expected from swine flu.
"I think what the American people need to be confident of is that President Obama, the Department of Homeland Security and the Centers for Disease Control and Prevention in Atlanta are being very aggressive in tackling the problem," Valerie Jarrett, senior White House adviser, told "Good Morning America" today.
"We were unfortunately expecting that there would be deaths, but we're working really hard to educate the American people so that we take reasonable precautions."
The White House has already requested a billion and a half dollars to fight the growing outbreak.
Meanwhile, Mexico's Health Minister Jose Angel Cordova said on Tuesday night that more than 1,300 people were in hospitals, some of them "seriously" ill, out of a total of around 2,500 suspected cases of the virus.
"In the last few days there has been a decline (in cases)," Cordova told a news conference. "The death figures have remained more or less stable."
Cordova said the victims ranged from children through young adults and middle-aged people to the elderly, a different pattern to the common seasonal flu that mainly kills the elderly and already unwell people.
British Prime Minister Gordon Brown said today three more cases of the flu have been confirmed, for a total of five known cases.
In the U.S., the outbreak has escalated, with the total number of cases confirmed in the United States jumping to 91 and spreading to more states, health officials said. Experts expect that figure to keep rising.
"I do expect more cases and expect more states to be affected," Rear Admiral Dr. Anne Schuchat, interim deputy director for the Centers for Disease Control and Prevention's science and public health program, told a Senate hearing today. "I think we need to be prepared that even if it starts to look a little better, it may get a little worse."
The CDC has been working to distribute test kits for the virus, identified only last week in the country. Without the kits, confirming infection can take days.
To fight the epidemic, the Obama administration is asking Congress for $1.5 billion. White House press secretary Robert Gibbs said the president was requesting the funds "out of an abundance of caution" to "enhance our nation's capability to respond to the potential spread of this outbreak."
The government's request underscores how seriously U.S. officials are treating the threat. Alarmed by the growing number of swine flu cases and the possibility that a California man may have died from the disease, Gov. Arnold Schwarzenegger declared a state of emergency today to help deal with the outbreak.
California has confirmed 15 cases of swine flu, and Tuesday it was investigating whether one man may have died as a result of the virus. According to Los Angeles County public health official Jonathan Fielding, the case is still being investigated. If it is confirmed, it would be the second death in the U.S. attributed to the illness.
Besser declined to comment on the California investigation, but said, "I fully expect we will see deaths from this infection."
Florida has reported an incidence of swine flu today, and Indiana health officials confirmed this afternoon that a student at Notre Dame has swine flu. These developments bring to seven the number of states where the infections are being investigated.
In Orlando, Fla., the chief medical officer at Florida Hospital Loran Hauck indicated the flu has spread to a tourist who visiting the crowded Disney World.
"A case was diagnosed here in Orlando today on a tourist from Mexico who came to Disney attractions two days ago to visit," Hauck wrote in the email. Florida health officials have not confirmed that the tourist was stricken with swine flu.
The outbreak is expected to keep growing because the CDC said today the new count includes "a number of hospitalizations."
The CDC in Atlanta has become the center for the swine flu investigation. It is one of only four laboratories in the world that have the expertise to unravel a novel flu strain.
"What we're trying to do is to identify how bad, how good, the swine flu is currently operating -- is it expanding, is it contracting, is it maintaining a steady state," said Phillip Navin, director of the Division of Emergency Operations at the CDC.
With the uptick in cases, the CDC shipped out more anti-virals and sent additional teams into the field.
"I think at the moment we need to be looking forward and making sure that we're doing everything that we can to keep people from getting sick," said Dr. Steve Reed, director of the CDC's Influeza Coordination Unit.
The biggest increase occurred in New York City where the number of cases leaped from 28 to 45, the CDC said.
Mayor Michael Bloomberg said Monday the cases were confined to a "single cluster," students of St. Francis Preparatory School and their relatives. Several of the students had spent spring break in Cancun, Mexico, which health officials suspect is the origin of the outbreak.
But Bloomberg said Tuesday that it appears to have spread to at least one other school, Public School 177, a school for autistic children. Of PS 177's 380 students, 82 called in sick today and at least a dozen have fevers. One of those students has two siblings at St. Francis Prep, Bloomberg said. Both of the those schools are located in the borough of Queens.
The mayor also said there are six possible cases of the flu in a Manattan school, Ascension parochial school.
And New York City Health Commissioner Thomas Frieden said today that "many hundreds" of students and teachers at St. Francis are sick -- though most of those cases appear mild so far.
"It is here and it is spreading," Frieden said. "We do not know whether it will continue to spread."
At one point the Ernst and Young corporation said it had confirmed a case at its Manhattan headquarters, but later retracted the statement.
In addition, there were four more cases in Texas and three additional cases in California.
Leaders in Congress raced each other to hold hearings on the outbreak. Sen. Tom Harkin of Iowa, chairman of the Senate appropriations subcommittee that deals with pandemic preparedness, called an emergency hearing to address funding for states and federal government
Sen. Joe Lieberman, I-Conn., called another hearing for Wednesday and Rep. Henry Waxman, D- Calif., the chairman of the House Energy and Commerce Committee called for a hearing on Thursday.
Concern among travelers was so great that Carnival Cruises Tuesday suspended its stops at Mexican ports because of its concerns about swine flu.
Around the world, New Zealand confirmed 11 cases, and Israel confirmed one Tuesday. South Korea, Australia and the Czech Republic announced several suspected cases.
A South Korean Catholic nun traveling in Mexico has tested positive for swine flu, according to reporting by ABC's Joohee Cho. South Korean authorities believe the 51-year-old woman caught the disease from a taxi driver and have requesting the Mexican government look into the matter.
Ten countries have restricted their imports of U.S. pork or swine, including Russia, China, Philippines, Thailand, South Korea, Kazakhstan, Serbia, Ukraine, the United Arab Emirates and Ecuador.
U.S. Agriculture Secretary Tom Vilsack said pork, soybean and corn prices had fallen in the last two days and criticized what he said were illogical restrictions on pork.
Korean health authorities are examining 315 other passengers that were on the same flight with the nun and have already injected Tamiflu into eight passengers who sat near her.
Spain had one previously confirmed case, the United Kingdom, two and Canada, six. France is still testing some people and some test results in Germany came back negative.
Most of the individuals involved had recently returned from Mexico.
To slow the global spread of the virus, the U.S. State Department and the European Union's health commissioner recommended avoiding nonessential travel to Mexico.
Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus amid global concern about a possible pandemic, which means a prevalent and rapidly spreading disease over a large region.
Singapore's Minister Mentor Lee Kuan Yew told the Straits Times that Singaporeans returning from Mexico would be quarantined.
Argentina also installed airport heat sensors and suspended flights from Mexico, as did Cuba.
But Gregory Hartl, a World Health Organization spokesman, told reporters that it did not recommend border closure or travel restrictions as a countermeasure.
He noted that infected people may not show symptoms at the airport or when they reach a border crossing. "Certainly, if you feel that you are ill, you should not travel, in any case, to anywhere," Hartl said.
Lessons learned from the SARS (a viral respiratory disease) outbreak in Asia in 2003 showed that border closure was economically disruptive and not particularly effective. "In public health terms, it didn't work, so we don't want to repeat something that didn't work" Hartl said.
But governments in Asia were not taking any chances. Singapore, Thailand, Japan, Indonesia and the Philippines dusted off thermal scanners used in the SARS crisis as they checked for signs of fever among passengers from North America.
South Korea and Indonesia introduced similar screening. In Malaysia, health workers in face masks took the temperatures of passengers as they arrived on a flight from Los Angeles.
China said anyone experiencing flulike symptoms within two weeks of arrival had to report to authorities.
India will start screening people arriving from Mexico, the United States, Canada, New Zealand, Spain, Britain and France for flulike symptoms, said Vineet Chawdhry, a top health ministry official. It also will contact people who have arrived from Mexico and other affected countries in the past 10 days to check for the symptoms, he said.
These measures came after the World Health Organization announced that it had raised its pandemic alert level to Phase 4 from Phase 3, the first time the alert level has been raised above 3 since the system was adopted in 2005.
The Phase 4 designation signifies that the new swine flu virus can cause sustained outbreaks and is adapting itself to spread among humans, significant steps toward a pandemic. But the ranking does not necessarily mean that a pandemic is a foregone conclusion.
"If the virus is an efficient virus, if it spreads easily from human to human, it will probably continue to spread," Hartl said.
All transmission of the disease so far appears to have been human to human and not from animal or other contact, according to the WHO. "There is no danger from eating pork," Hartl said. "If you cook pork well, if you cook all meat well, it kills all virus."
Outside Mexico, where 159 people have died, the United States has been the country worse hit by the swine flu outbreak. Many of the cases appear to be students who recently visited Mexico.
Addressing the National Academy of Sciences Monday, President Obama acknowledged the efforts to keep pace with the spread of the disease within the United States.
"We are closely monitoring the emerging cases of swine flu in the United States," he said. "And this is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm."
There is no vaccination for the swine flu strain, which has elements of pig, bird and human strains. But officials said they have ramped up medical surveillance around the country and, as part of the emergency declaration, freed up state and federal resources for prevention. Officials also emphasized the importance of individual care and good hygienic practices.
"Even if this outbreak is a small one, we can't anticipate we won't have follow-up outbreaks," U.S. Homeland Security Secretary Janet Napolitano said.
A different scenario is unfolding across the border in Mexico. While authorities hunt for the source of the swine flu outbreak, the country is under lock down. Schools, museums, parks and even churches in Mexico City have been shut down by the government. A leading business group estimates that canceled events and closure of establishments to prevent the spread of swine flu is costing Mexico City at least $57 million a day.
All of Mexico's Mayan ruins and Aztec pyramids have also been shut down until further notice.
"I haven't been out for days," said one woman, who only left to bring her baby to the doctor for a routine vaccination.
The government has advised people to stay home and indicated that those infected by the virus could be isolated. In the country's deserted capital, public events were cancelled for the next week or so. Sales of masks have soared as people try to prevent themselves from the potentially deadly disease.
Mexican officials are hoping the 10-day shut-down will be enough to cover the two-day incubation period and the seven-day recovery of anyone who has the virus. On Wednesday, however, the Associated Press reported that Mexico City Mayor Marcelo Ebrard said the swine flu situation in the city seems to be stabilizing, with only one more death since he last spoke. The AP further reported the mayor as saying that if the death toll continues to taper off, he will consider easing the shutdown of the city.
Mexico's first suspected case of the swine flu was detected in the remote farming village of La Gloria, where 5-year-old Edgar Hernandez contracted the disease nearly one month ago, authorities say.
"The most likely way that this young boy got the infection was from another person who had been in contact with the pigs," said Dr. Kathryn Edwards of Vanderbilt Medical Center.
More than 800 people in the town of 2,000 were infected, authorities say, but no deaths were reported. It took seven days for Mexico to confirm its first cases of swine flu, according to World Health Organization estimates.
Officials say it's still too early to determine how the disease spread from La Gloria into a global health emergency.
"It's a new virus, new virus combination, it does transmit from person to person and we already know it causes fatalities so we already have all the makings of a possible pandemic," Irwin Redlener of Columbia University's Mailman School of Public Health said.
But Dr. Nancy Cox of the CDC has said she believes the earliest onset of swine flu in the United States in this current outbreak happened March 28.
A quarter of the 50 million doses of Tamiflu stockpiled by the U.S. government has been released and the Obama administration has declared a public health emergency to free up the medicine and federal help to the states who need it.
But pharmacies in several states have been flooded with phone calls from concerned customers.
"Our first phone calls were doctors asking if we had Tamiflu," New York City pharmacist Yvonne Zampitella said. "They were prescribing it for their patients and family members."
Symptoms of the swine flu are similar to the regular flu, health officials say, including aching muscles, fever and fatigue. The virus appears to be responsive to medication.
"These drugs do not kill the virus, they help prevent its replication and therefore help reduce the symptoms, but they have to be taken within 48 hours so people have to recognize they have a serious illness, get to a doctor and start treatment," ABC News' Dr. Tim Johnson said on "World News" Monday.
"But we should not be telling people to go out and buy these drugs for use as preventive measure. We need to reserve drug for actual cases and outbreak."
The Associated Press contributed to the reporting of this story.