The top U.S. public health official said today there are encouraging signs that the swine flu virus seen in this country will be less deadly than previous flu strains -- but nevertheless, the number of cases of the H1N1 flu virus continues to grow around the globe.
The Centers for Disease Control and Prevention officially confirmed 226 cases in 30 states this morning, but during the day, North Carolina, Pennsylvania, Oregon and Louisiana joined the list for a total of 34 states and 245 cases.
Globally, officials in 19 countries have confirmed at least 917 cases -- slightly more than the 898 in 18 countries confirmed by the World Health Organization.
Officials in Mexico said there have been more than 500 confirmed cases of the disease and reported three swine flu deaths late Saturday, bringing the death toll in that country, believed to be the epicenter of the outbreak, to 19. In the United States, the virus claimed the life of a toddler in Texas last month.
Mexico's Health Minister Jose Angel Cordova said today that "the evolution of the epidemic is now in its phase of descent," but officials in the United States and at the World Health Organization in Geneva said that the world must remain vigilant.
WHO epidemic and pandemic diseases spokesman Gregory Hartl said today that though the current epidemiology in Mexico might show a slowdown, history must be considered when evaluating the virus' potential to come back.
"I also would like to remind people that in 1918, the Spanish flu showed a surge in the spring and then disappeared in the summer months only to return in the autumn of 1918 with a vengeance, and we know that killed eventually 40 to 50 million people," Hartl said. "So I think while tracing these kind of curves of activity, increasing, decreasing activity, we cannot lower our guard."
On "This Week," acting CDC director Dr. Richard Besser said that "we're seeing encouraging signs" that H1N1 is less virulent than past strains.
As for the CDC's analysis if the H1N1 virus, Besser said, "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."
But though officials are cautiously optimistic, whether the virus will reemerge 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 this country.
Pandemic Alert Level Depends on H1N1 Virus Spread
The WHO's current pandemic alert level stands at phase five, which "is a strong signal that a pandemic is imminent," according to the organization's guidelines. During the current phase, officials around the world have stepped up preparations for a potential pandemic and increased communication and coordination.
On Saturday, the director of the WHO's global alert and response team, Dr. Michael J. Ryan, proposed that a pandemic is "imminent, because we are seeing the disease spread."
Hartl added today that a time frame for an upgrade in alert level "is not very easy to predict, because transmission of influenza virus, or of any virus for that matter, does not necessarily always move at the same speed. There could be hiatus in its spread, especially [because] we have not seen any confirmed instances of sustained human-to-human transmission in communities outside of the Americas."
"We don't know when that will happen; certainly we remain on alert in case that happens," he said. "But ... it would not be prudent to try to make a prediction" on the timing of a potential decision to step up the alert level.
The next level, six, would indicate that "a global pandemic is under way," the guidelines say. Ryan cautioned Saturday that the term pandemic only refers to the geographic spread of a disease, not its severity.
Swine Flu Concerns: Safe to Eat Pork?
There have been concerns about the safety of eating pork since the outbreak of the virus, though officials have maintained that the meat is safe. Global health officials have ceased calling it swine flu in favor of its scientific designation, H1N1, as part of the effort to reinforce that notion.
But on Saturday, concerns grew after Canadian officials confirmed that pigs on a farm in that country tested positive for the virus. The farm has been placed under quarantine.
WHO food safety scientist Dr. Peter Ben Embarek said today that a farm worker who had recently visited Mexico likely brought the virus to the farm, but that the humans and animals infected in that case are recovering.
Embarek said the virus infected only about 10 percent of the herd of 2,200 pigs.
"This tells us that, also for the animal population, it doesn't seem to be a very serious disease," Embarek said. "And it's apparently, from what we know, something that is not surprising and could be expected from a virus like this."
He added that, though farm workers and those who slaughter and process infected animals could be at a greater risk for picking up the disease, "from the consumer point of view, there is absolutely no risk of consuming cooked products", and that the meat trade should not be restricted.
Obama: U.S. Acting 'Quickly and Agressively' to Combat H1N1
President Obama said in his weekly address to the nation Saturday that the United States is acting "quickly and aggressively" to prepare for a potential pandemic, and is "investing in every resource necessary to treat this virus and prevent a wider outbreak."
His newly-confirmed secretary of health and human services, Kathleen Sebelius, said on "This Week" that the government is taking action to develop a vaccine for the H1N1 virus in addition to preparing for the fall flu season.
"The good news is we're in the right seasonal time. We can accelerate the seasonal flu vaccine which we're doing right now, to be prepared and ready for what we know will hit this fall and winter," Sebelius said, "At the same time, we are in the stages of growing the virus, testing it, and we can be ready to do both simultaneously."
Obama added the nation's stockpile of antiviral medications has been opened up to the states, with a quarter of the supply of 50 million courses of treatment already released. The U.S. government also purchased 13 million treatments to replenish the stockpile, he said.
The WHO said Saturday that it released 2.4 million antiviral courses to 72 developing countries, including Mexico.
Obama added that "out of an abundance of caution," he has asked Congress for $1.5 billion to be set aside for more medicines, supplies and the development of an H1N1 flu vaccine.
Officials continue to caution those who feel ill to stay home, for individuals to wash their hands frequently and for schools with a confirmed case of the virus to close for up to 14 days, with frequent reevaluation of their particular situation.
Schools are of particular concern, the CDC's Dr. Anne Schuchat said Saturday, because children take longer to shed viruses and other diseases, and often do so without symptoms. The majority of the swine flu cases in the United States have occurred in people younger than 20 years old. The median age of those with the disease is 17, though the ages range from 1 to 81, according to Schuchat.
"The goal" of social distancing efforts like school closures, Schuchat said, "is to try to decrease the ongoing numbers of cases and to shift them to a later time by slowing transmission."
This weekend, religious organizations are altering their services to reduce close contact between congregants. Some have made the decision not to place communion wafers into individuals' mouths or to serve wine, an attempt to limit the risk for transmission of the disease.
ABC News' Gregory Croft, Ayana Harry and wire reports contributed to this report.