In a widely anticipated emergency meeting today in response to the swine flu threat, the World Health Organization announced that it is raising 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.
To slow the spread of the virus, the State Department issued a travel alert this evening, recommending that Americans avoid any non-essential travel to Mexico. In addition, the Food and Drug Administration has authorized public health and medical officials to use influenza medications, like Relenza and Tamiflu to treat the virus and deal with the emergency situation.
Meanwhile, 20 additional cases of swine flu have been confirmed at a New York City school, boosting the total number of cases there to 28, New York City Mayor Michael Bloomberg said today.
Bloomberg warned that there are another 17 possible cases, all in people associated with the school, St. Francis Preparatory School in Queens, and as many as 100 of the students have fallen ill and will be tested for the virus.
Some of the school's students had recently returned from a spring break trip to Mexico.
The mayor expressed optimism, however, over the fact that the disease has not yet been detected anywhere else in the city.
"We are still dealing with a single cluster of swine flu cases, all associated with this one school," Bloomberg said, adding that to his knowledge all of those infected with the virus were improving.
"So far, we are not seeing a situation comparable to that being reported in Mexico."
The total number of swine flu cases throughout the U.S. rose to 50 today, The Associated Press reports. The challenge confronting public health officials around the world is how to stop a disease from spreading when it travels by human contact.
Today the Centers for Disease Control and Prevention announced a new travel advisory for Americans visiting Mexico, according to Dr. Richard Besser, acting director of the federal agency.
Besser made the announcement this afternoon at a press conference.
While swine flu cases in the United States appear to be less severe than those in Mexico, and no deaths have yet been reported, U.S. health officials warned that the outbreaks could get worse, asking Americans to avoid any non-essential travel to Mexico.
"What I want people to understand is that we're concerned," Besser told "Good Morning America" today. "People should be concerned. We'll tell you what we know when we know it."
The United States has not reported any swine flu fatalities, Besser said Americans need to be prepared for it to get worse.
"I think from what we understand in Mexico, people need to be ready that we can see more severe cases in this country," he said.
Addressing the National Academy of Sciences on Monday, President Barack 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."
The virus is no longer just a North American problem. With Europe's first confirmed case of swine flu in Spain, the European Union's health commissioner has warned Europeans to avoid nonessential travel to Mexico and the United States. There are also suspected or confirmed cases in the United Kingdom, France, Israel and New Zealand; six confirmed cases in Canada – all among those who recently traveled to Mexico.
Meanwhile, in the United States, the CDC has been trying to quell American's fears about the disease that may have already claimed the lives of 149 people in Mexico, according to the country's health minister. Nearly 2,000 people have been hospitalized since mid-April and only half have been released. Some say the danger of a pandemic is real.
Mexico's first suspected case of the swine flu was detected in the southern state of Oaxaca, Health Minister Jose Angel Cordova said Monday, according to Reuters. He said it was too early to identify the cause or geographical source of the disease.
"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 of a potential severe outbreak that spreads to a wide geographic area.
The CDC's Besser said that although the swine flu is highly contagious, there are simple precautions Americans can take to stop the spread of the virus: washing their hands, staying away from crowded places and staying home if they feel ill.
Because there have not been any deaths in the United States, so far authorities have said the virus may have mutated into a weaker form after first spreading from pigs to humans. Besser said officials in the United States are trying to get more information from Mexican authorities to determine why their cases have been more severe.
Hayden Henshaw's family has been quarantined in their Cibolo, Texas, home after the teenager caught the swine flu. His father and sister later came down with the virus, showing how fast it can spread.
"My muscles hurt, my skin hurt, I couldn't pick my head up out of bed," Henshaw said.
Robin Henshaw, Hayden's mother, is the only member of the family not to come down with the swine flu.
"I'm just washing my hands a lot," she said. "We're using a lot of Lysol and we're taking Tamiflu."
While Mexico City -- considered to be this outbreak's ground zero -- has been largely deserted after an order came to effectively shut the city down for the next 10 days, the United States is taking swift action to ensure the flu doesn't cause similar problems here.
Border crossings and airports are the frontlines of the battle to halt the spread of the disease. More than 13 million people traveled from Mexico to the United States last year, according to the Department of Commerce, and 20 million Americans traveled to Mexico last year alone.
At the San Ysidro border crossing, where about 80,00 people cross daily, custom agents are on the lookout for those who may be sick -- taking those in question to a secondary holding facility for evaluation.
At airports, US Airways, American Airlines and Delta all reported cancellations and the airlines waived fees for Mexico travel.
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," pharmacist Yvonne Zanpaitella 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 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," said ABC News' Dr. Tim Johnson on "World News." "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."
Overseas, authorities are taking extreme measures to prevent a pandemic. Still scarred by memories of SARS and bird flu, in Japan and China, they're using technology to measures a passengers' body heat, quarantining those arriving from the United States and Mexico with fevers.
In Beijing, thermo graphic cameras and infrared scanners are being used to monitor passengers for a high temperature. Anyone displaying signs of a fever will then be checked by officers and could be quarantined.
In Mexico City, this normally bustling city of 20 million people has been reduced to a virtual ghost town. Schools, museums, parks, even churches have been shut down by the government.
"I haven't been out for days," said one woman, who only left to bring her baby to the doctor for a routine vaccination.
But at least the airport is still open.
"I'm just crossing my fingers that nothing happens in the next four hours and I can get back to the states," said Drew Carlisle, a U.S. tourist from Birmingham, Ala., as he waited for a flight home.
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.
Mark Powell, a photographer from Detroit, lives here with his wife and two children. They can't go out, but are going stir-crazy inside.
"Their schools are canceled so we have to plan some indoor activities, but also you can't invite other kids over," he said. "Shaking peoples' hands, you feel a little self-conscious -- or the traditional kiss in Mexico, a little peck on the cheek."
U.S. Department of Homeland Security Secretary Janet Napolitano said Sunday the government declared a public health emergency, which will allow it to free up resources to tackle the issue.
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," Napolitano said.
In Mexico, more than a thousand people have been infected. The Mexican government has advised people to stay home, and the government has indicated those infected 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.
The World Health Organization is considering raising its threat level from three to four. But while it characterized the situation as serious, officials said they are not yet convinced that the swine flu is a pandemic -- meaning that it is easily transmitted from person to person and can cause large outbreaks across the world.
"If we move the pandemic threat, want to make sure on pretty good solid ground. Such a move would be a big signal to the world. We already have several countries involved (with positive cases), but we know in current global situation that cases can occur in many places without ever taking hold. We have decided to wait and get more information," said Dr Keiji Fukuda, assistant director-general ad. interim for Health Security and Environment at WHO.
WHO officials said so far, the virus has only been confirmed in Mexico and the United States, and that the transmissibility is limited and the outbreak is small. WHO officials said are working on a vaccine and testing and identifying this particular never-seen-before virus.
The severity of the flu is still unknown. Some U.S. doctors say they do not trust information from Mexico, but WHO officials said they do not believe that Mexico created any delays in reporting the outbreak.
"We only have isolated reports from various places. You have to have more than this" to declare a pandemic, said Peter Katona, an associate professor at the University of California, Los Angeles (UCLA) Medical Center. "This virus may burn itself out tomorrow. We don't know that yet."
American Airlines, US Airways and Continental are waiving fees for flight changes to Mexico. They are not cancelling flights, and American Airlines said it's only received a handful of calls seeking changes in travel plans.
Katherine Andrus, an attorney for the Air Transport Associated, told ABC News that the organization is taking this seriously and employing prudent measures, but at this point it would be an extreme measure to restrict any flights.
Dr. William Schaffner, chairman of Preventive Medicine at Vanderbilt University Medical Center in Nashville, Tenn., said doctors like him have been advised by the CDC and state health department to set up a system that would test patients with flu-like symptoms and help define how widespread this outbreak is. He said the severity of the virus is hard to gauge because of the wide discrepancy in how it has affected Mexicans and Americans, and because it is occurring in places that are warm, which is very unusual.
"The genetic make up of this virus has influenza experts scratching their heads," he said. "One of the things that has us worried is that could this be a virus that could continue to make mischief during the warmest parts of the year. That would be a big thing. For a respiratory virus to be active during the summer months" would be very unique.
The escalation in the swine flu situation on Saturday underscored concerns by international, federal and local health agencies over the threat of the new virus, even as government health officials said much remains unknown.
But if one thing is clear about the spread of this virus, it is that containment is no longer an option.
Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases said broad spectrum of the illness is expected in the United States.
"It's clear that this is widespread," said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, at a press conference Saturday afternoon.
"We do not think that we can contain the spread of this virus," Schuchat added. "Having found virus where we have found it, we are very likely to find it in other places. ... We are not at a point where we can keep this virus in just one place."
The cases further demonstrate that health officials do not know where else the virus might turn up. Details of swine flu's spread in Mexico also remain murky. A team sent by the Centers for Disease Control and Prevention is now on the ground there seeking clues on the origin and spread of the disease.
"What we still don't know is how widespread it is," said Schaffner. "The question is: How long have things been going on in Mexico, and how attentive have they been in terms of what's going on in their country?"
Schaffner added that had it not been for the death cases in Mexico, the swine flu would have largely gone unnoticed in the United States.
"My observation has been, were it not for the problem in Mexico, this would have been on page 15 instead of page 1," he said. "Because each year we have dominant influenza strains but we also have occasional strains that don't fit the dominant patterns."
ABC News' Matt Hosford contributed to this report.