Amid confirmation that 20 Americans have been diagnosed with the swine flu, officials at the U.S. Centers for Disease Control (CDC) said they fear death in the United States as they expect more cases of the virus to emerge.
Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said it is premature to say the disease in the United States is different than the one that has killed about 86 people in Mexico, and how serious this new virus is compared to typical flu viruses remains to be seen. On Sunday, Canada became the third country to confirm human cases of the unique virus.
Earlier today, Department of Homeland Security Secretary Janet Napolitano said the government declared a public health emergency, which will allow it to free up resources to tackle the issue.
At a White House press conference, Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention (CDC), ensured Americans that the administration is taking aggressive steps to control the outbreak, but that more cases are likely to expect to emerge in the near future.
"Flu viruses are very unpredictable. Outbreak of infectious diseases are very unpredicatable," Besser said. "We view this more as a marathon. We think this will continue to spread but we are taking aggresive action."
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 hygenic pratices.
The news came on the heels of New York Mayor Michael Bloomberg's announcement that eight students at St. Francis Preparatory School in Queens were confirmed to be infected with the swine flu. About 100 students reported flu-like symptoms at the school, which will be closed on Monday and Tuesday.
"The numbers here are not important, what's important is that at the school we have a confirmed cluster of swine flu," New York Health Commissioner Thomas Frieden said. "It's obviously concerning to have a new strain. It's obviously concerning the size of the number of students affected at the school ... However, it's reassuring that every case ... has been mild. Many are already improving and with careful looks at the intensive care units in the city we have not identified an increase in severly ill people who may have this infection."
Eight people have been confirmed diagnosed in New York City, one in Ohio, two in Texas and seven in California.
Officials dusted off the idea of a bio-terrorist threat. "There is nothing we have seen in our work that would suggest anything but a naturally occuring event," Besser said.
Besser confirmed that the virus found in Americans is the same as that in Mexicans, but that they have not been able to determine why the impact of the flu has been more severe in Mexico, where about 86 people have died from the disease. In the U.S., only one person infected had to be hospitalized. Officials stressed they are taking an aggressive approach to tackling the issue -- they have released 12.5 million of the nation's stockpile of 50 million courses of Tamiflu, a drug that has shown itself at least initially to be effective against the flu virus -- but used cautious language in descrbing whether it could be contained.
"Even if this outbreak is a small one we can't anticipate we wont have follow up outbreaks," said U.S. Homeland Security Secretary Janet Napolitano.
In Mexico, more than a thousand people have been infected. The Mexican government has advised people to stay homebound, 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.
"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 transmissability 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."
Several countries have reportedly announced measures to test American visitors and imposed travel warnings for Mexico. The United States has not yet imposed a travel advisory to and from Mexico, with officials only suggesting to Americans to reevaluate their plans if visiting the country.
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 flulike 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 occuring 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."
Meanwhile, emergency departments in affected areas are preparing for what they said could be a rise in cases.
"It is quite conceivable that it could escalate fairly rapidly," said Dr. Richard Bradley, chief of the Division of EMS and Disaster Medicine at the University of Texas Health Sciences Center at Houston. "If we are seeing a flu epidemic in Mexico, we should expect an uptick fairly quickly in Texas, where I am, and in California."
Still, Bradley said that the situation thus far has been relatively fortunate.
"Right now, we are in the increased awareness phase -- we don't yet know how this is going to develop," he said. "Fortunately, right now the cases in California and in Texas seem to be milder than the cases we've seen in Mexico. We hope that this trend continues in the U.S."
Emergency departments in other areas of the country also are stepping up efforts to control the virus should it surface.
Dr. Lewis Goldfrank, professor and chairman of the Department of Emergency Medicine at Bellevue Hospital in New York City said that though no cases have been detected in his emergency department yet, any patient who comes in with a cough and a fever or a rash and a fever is isolated for a nasal swab to detect influenza -- a test that takes about an hour.
The possible outbreak in the New York area suggests the measures may be warranted.
Another question that remains is whether health agencies reacted quickly enough to the initial reports of swine infections in Mexico, first reported in mid-March.
"I think that the laboratory testing capabilities for this type of virus were not fully in place and this accounts for some of the delay," said Dr. Pascal James Imperato, dean of the Graduate Program in Public Health at SUNY Downstate Medical Center in Brooklyn, N.Y. "They were not routinely testing for this type of influenza virus."
Still, other infectious disease experts said that there should have been quicker action.
"I concur that the 'one-month lag' between case identification and reporting was too long," said Ed Hsu, associate professor of Public Health Informatics at the University of Texas School of Health Information Sciences and School of Public Health. "[It is a] good thing that the CDC now decides to have a daily briefing on the swine flu, but it will still need to bring the transparency of reporting in other high-risk countries up to speed on this."
And despite the actions by the World Health Organization with regard to the swine flu outbreaks, no decision has yet been made to increase the pandemic threat level from its current status of phase 3 to phase 4 on the six-point scale. A virus isn't considered to be pandemic until it reaches phase 6.
WHO did announce, however, that it has sent a team of experts to Mexico to further study the outbreaks.
Despite the action being taken by national and international health organizations, some infectious disease experts say it is far too early to fear the worst -- a global flu pandemic.
"The current swine flu only marginally meets only one of three of pandemic tests: effective person-to-person transmission," Hsu said, adding that the other tests -- susceptibility and no natural immunity or vaccine -- remain to be satisfied.
Hsu further noted that compared to the H5N1 strain of the avian flu virus -- commonly known as bird flu -- the current H1N1 swine flu strain is still a relative lightweight. Since 2003, he said, bird flu has garnered a 60 percent case fatality rate, and it never attained pandemic status. Meanwhile, the current swine flu strain still has fewer than 1,000 reported cases and only about 60 fatalities to its name.
"If not [a pandemic] then, why now?" he asked.
"I think its still early to say," said Nathan Wolfe of Global Virus Forecasting Initiative. "We're still early on in an outbreak like this. One of our main objectives is to try to figure out case mortality rate, what percentage of individuals that it affects does it kill. But what's really a concern is that this virus is clearly spreading from human to human, which is at the point where we become really interested and focus on understanding that."
But if more cases did arise, Schaffner said that a vaccine for the illness would not be available for months.
"It would be an Olympic sprint for vaccine manufacturers, starting today" to have a usable vaccine ready even by October, he said.
"If this is a virus that is sufficiently new -- and that has not been entirely determined yet -- we may need two doses of the vaccine to get protection," Schaffner said. "That, of course, would put additional strain on the vaccine production and delivery services."
ABC News' Dan Childs and Matt Hosford contributed to this report.