The CDC reports that of those hospitalized with H1N1 swine flu since Sept. 1, 53 percent were under the age of 25, 39 percent were ages 25 to 64 and only 7 percent were 65 or older.
So how do you know if it's really the flu and not some other respiratory illness?
Seattle virologist Dr. Anna Wald says there are definite symptoms.
"Sudden onset, high fever, headache, myalgias or body ache, however in this particular strain we have seen quite a bit of abdominal pain," she said.
When to call the doctor? Dr. Wald's recommendation is to err on the side of caution.
"Influenza is a potentially a very severe illness and it's not infrequent for people to be hospitalized for it and to receive anti-viral drugs and that is the reason why I would urge people who are really ill to seek care because the currently available anti-viral drugs make a substantial difference, but they only make a difference if they're taken early in the course of the disease," she said.
When to call 911 instead?
If you have chest pains, trouble breathing or turning blue seek immediate medical help.
That applies especially for pregnant women, the obese and children who are more likely to suffer life-threatening complications.