The White House says 20 to 40 percent of Americans will come dow with the swine flu this fall.
Officials also caution that here may be more deaths than is typical from seasonal flu. This is making the need for a vaccine to combat this strain of flu even more urgent.
Health experts predict that the first round of vaccinations will not be effective before Thanksgiving. It takes five weeks or more of regular dosing before immunity kicks in.
Clinical trials to evaluate the new vaccine began in August. Vaccine makers expect to have 50 million doses ready by mid-October.
Now children of all ages, from grade school to college age, are being bombarded with health messages as they return to school.
Health officials said the swine flu is unlike other flus, because it is persisted throughout the summer.
"This isn't the flu that we're used to," said Kathleen Sebelius, U.S. Secretary of Health and Human Services. "We have seen transmission of a virus in a season that should not be flu."
The swine flu is also unusual because it has not yet affected seniors, and seems to be targeting children.
"We know it spreads rapidly among otherwise healthy kids," Sebelius added. "Unusual for a flu."
Even more sobering, a White House report on Monday estimated the virus will cause between 30,000 and 90,000 deaths. That is a potentially much greater number than seasonal flu, which kills an estimated 36,000 every year.