Flu season has started, and although so far it has not been as bad as last year's, there have been reports of some young and middle-age adults developing severe cases of influenza, according to the Centers for Disease Control and Prevention.

Today (Feb. 1), the CDC announced that flu cases are increasing across the country. And although the nation as a whole isn't experiencing as much flu as this time last year, "some localized areas of the United States are already experiencing high activity, [and] further increases are expected in the coming weeks," the CDC said in a health alert to physicians.

The most common flu strain circulating now is H1N1, the same strain of flu that caused a pandemic in 2009.

What's more, the CDC has received reports of severe flu illness developing in young to middle-age adults who are infected with H1N1. Some of these people needed to be admitted to the intensive care unit, and some died, the agency said. Most of these patients hadn't been vaccinated with this year's flu shot. In the past, H1N1 has been known to cause particularly severe disease in younger adults.

The CDC urged doctors to use antiviral medications as soon as possible for everyone hospitalized for suspected flu, and the agency said doctors don't need to wait for a test to confirm that an infection is indeed the flu before beginning the treatment. [6 Flu Vaccine Myths]

Overall, most U.S. states are reporting minimal flu illness so far this season, but three states — Arizona, Maryland and South Carolina — are reporting moderate flu activity, and Puerto Rico is reporting high activity.

In addition, from Jan. 10 to Jan. 16, 2.1 percent of people visiting the doctor were there because of a flu-like illness. The "national baseline" for flu visits — the threshold for what's typically seen in the off-season — is also 2.1 percent. One marker for the start of flu season is when the percentage of doctor's visits for flu reaches or exceeds the national baseline, the agency said.

So far, the hospitalization rate for flu is 1.8 hospitalizations per 100,000 people in the United States. That's much lower than the hospitalization rate at this time last year, when the rate was 36 hospitalizations per 100,000 people.

But the CDC noted that reports of hospitalizations and deaths from flu typically lag behind reports of increases in doctor's visits for flu. "CDC will continue to watch for indications of increased severity from influenza virus infection this season," the agency said.

The CDC recommends that everyone 6 months and older receive a flu shot, and it's not too late to get one if you haven't already this season, the agency said.

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