The United States has 896 cases of the new H1N1 flu in 41 states, with two deaths, the U.S. Centers for Disease Control and Prevention reported Thursday.
Officials have said they expect the swine influenza virus to spread to all 50 states and to cause many infections ranging from mild to severe.
Meanwhile, a U.S. health official says now only about 10 percent of the Americans who got swine flu had traveled to Mexico and likely picked up the infection there. Most got the bug at home.
That's a change from over the weekend when the Centers for Disease Control and Prevention said about a third of the U.S. cases at that point were people who had been to Mexico, where the outbreak began.
The CDC's acting chief, Dr. Richard Besser, said Thursday that there are now nearly 900 confirmed cases. He says the ongoing spread within the U.S. borders explains why a shrinking proportion of cases are people who traveled to Mexico.
The ages of those who got swine flu now range from 1 month to 87.
The U.S. public could become more vulnerable to a flu pandemic if complacency about the need for heightened vigilance sets in, health experts said on Wednesday.
In all, there are more than 2,000 confirmed cases of the swine flu virus in 22 countries, according to the World Health Organization, which warned to Thursday that up to one-third of the world's population — about 2 billion people — may ultimately get infected with swine flu.
But in the United States fear about flu appears to have subsided since the epidemic came to public attention more than two weeks ago because many cases appear to be mild.
"The risk of complacency, or a sense that we have weathered this, is a serious one," said Stephen Redd, director of Influenza Coordination at the U.S. Centers for Disease Control and Prevention in Atlanta.
"What we are going to be looking very hard at over the months to come is what's happening in other parts of the world and really trying to understand whether we would be at risk for a resurgence in the fall," Redd said.
CDC officials say they walk a fine line between ramping up public warnings to encourage people to take precautions such as washing hands while not adopting an alarmist posture that could risk their authority as the epidemic persists.
In one small sign of waning public interest in the flu threat, nationally syndicated talk show host Neal Boortz told his audience recently that the issue of flu was getting "really overblown."
New York has seen more than 100 cases of H1N1 flu, though most have been mild, and Mayor Michael Bloomberg said he was aware of the need to calibrate warnings to the public.
"There is always that danger (of crying wolf)," said Bloomberg in an interview. "There is always the danger of over-reacting (but) government has the responsibility to tell people what's going on and what to do about it."
"When I start talking in New York about disease ... the first thing I say is: 'I'm going to tell you what we know and what we don't know. And I'm not going to speculate on anything between," Bloomberg said.
The CDC has spent years preparing for a pandemic and ranks communicating with the public on an equal footing with studying potential viruses and finding vaccines.
Dozens of scientists and public health specialists work amid a low volume of chatter at its 24/7 Emergency Operation Center, which is equipped with computer monitors, table lamps and hand sanitizer.
Teams of scientists collate and analyze data while others provide input on subjects ranging from ethics to policy to how to deal with the media.
The room is dominated by a series of flat-screen televisions set on one wall, each showing a critical piece of information and one tuned to CNN.
Elsewhere at the CDC, researchers are monitoring the virus to see if it could mutate into a more deadly strain.
They are conscious of a historic parallel — in 1918, a relatively mild flu pandemic emerged, only to return with a vengeance months later to kill millions.