"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.