Nearly half of the United States is reporting widespread influenza activity, most of it attributed to the H1N1 virus that caused a worldwide pandemic in 2009, the U.S. Centers for Disease Control and Prevention said on Friday.

Thousands of people die every year from flu, which peaks in the United States between October and March. The flu is spreading quickly this season, with 25 states already reporting cases, the CDC said.

“We are seeing a big uptick in disease in the past couple of weeks. The virus is all around the United States right now,” said Dr. Joe Bresee, chief of Epidemiology and Prevention in the CDC’s Influenza Division.

In 2009-2010, the H1N1 virus, also known as swine flu, spread from Central Mexico to 74 other countries, killing an estimated 284,000 people, according to the CDC.

While younger people were more susceptible to H1N1 in 2009, Bresee said it is too early to tell whether the same will be true this year.

This season’s virus has killed six children in the United States, according to CDC data. The agency does not track adult deaths, but dozens have been reported around the country.

“There is still a lot of season to come. If folks haven’t been vaccinated, we recommend they do it now,” Bresee said.

Texas has been one of the harder hits states, where at least 25 people have died this season from the flu, local health officials said.

The Texas Department of State Health Services issued an “influenza health alert” on December 20, advising clinicians to consider antiviral treatment, even if an initial rapid-flu test comes back negative. Texas health officials also encouraged people to get a flu vaccination.

“The flu is considered widespread in Texas,” Carrie Williams, a spokeswoman for the state’s health department, said.

(Additional reporting by Jon Herskovitz; Editing by Barbara Goldberg and Gunna Dickson)

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(Reuters)