MONDAY, May 11 (HealthDay News) -- As the swine flu continues to spread across the United States -- and most cases continue to be mild -- federal health officials said Monday that they're shifting their focus from individual cases of infection to trying to project what is likely to occur with the virus in the fall.

The U.S. Centers for Disease Control and Prevention is conducting field studies to "strengthen our knowledge about how this new virus is spreading, who is most at risk for illness, how effective prevention measures are, antiviral treatment and so forth," Dr. Anne Schuchat, the CDC's interim deputy director for science and public health program, said during an afternoon teleconference.

The CDC is concerned with what will happen as this new virus moves into the Southern Hemisphere, where the flu season is about to start. The CDC is also preparing for the virus' likely return in the fall to the Northern Hemisphere, Schuchat said.

Because the new swine flu virus -- technically called H1N1 -- is a highly unusual genetic mix of bird, flu and human viruses, health officials worry that it could continue to mutate and return in a more virulent form for next winter's flu season.

As of Monday, there were slightly more than 2,600 confirmed cases in 43 states and the District of Columbia, with three confirmed deaths and 94 people hospitalized, Schuchat said. All three patients who have died had underlying health problems before their infection with the flu. Some states do not report data over the weekend, so the number of new cases is likely to rise dramatically tomorrow, she said.

Schuchat added that the confirmed cases are likely just the tip of the iceberg. Many people who become ill don't seek medical attention and are never tested for this strain of flu. "The numbers we are reporting are a minority of the actual infections that are occurring in the country," she said.

The United States has now surpassed Mexico -- believed to be the source of the outbreak -- as the country most affected by the outbreak, the World Health Organization said Monday. The agency was reporting 4,694 confirmed cases of swine flu in 30 countries, with Canada, Spain and the United Kingdom having the most cases outside of the United States and Mexico.

While the swine flu is similar to seasonal flu, there are some important differences, Dr. Richard Besser, the CDC's acting director, said last week. "One thing we are seeing, unlike seasonal flu, a higher percentage seem to be having vomiting and diarrhea," he said.

Besser said most new cases of swine flu in the United States were now caused by person-to-person transmission and not some link to Mexico, as was the case when the outbreak began more than two weeks ago.