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JACKSONVILLE, Fla. -- Flu activity remains high in the U.S. and 23 counties in Florida are reporting an increase in cases of influenza.

UF Health in Jacksonville reports 256 patients have tested positive for the flu since the beginning of the season in 2013. Health experts say once you have the flu, you have a small window to treat it.

"Actually, I thought, I mean cause I had the flu shot, I thought it was a really bad cold or bronchitis or something like that and I didn't think it was the flu," said Donna Tressler.

Tressler is a billing manager for an Urgent Care facility in Mandarin. She along with other staff members got the flu vaccine, but it didn't make her immune to every strain.

"I had a really hot fever, " said Tressler. "My whole body ached."

Tressler became so sick she missed work for one week and had to be hospitalized for two days.

"All I did was lay. I had no strength to do anything. Every muscle in my body ached and I'm very hyper so that was very hard for me," said Tressler.

Luckily, her husband and 24-year-old son didn't get sick. Tressler stayed away from her grandchildren. She's one of hundreds testing positive for the flu on the First Coast.

"I've seen more this year than I have last year," said Ed McDonough, Physician Assistant at PMC Urgent Care. "I'd say we've seen probably a 25 percent increase maybe."

McDonough said at the Physicians Medical Center, he's seeing more severe symptoms.

"Based on when I walk in the room, sometimes I'll see them lay down on the table, sometimes they are just too sick to even get up," said McDonough.

Flu activity is widespread in 41 states according to the CDC. Florida has had two pediatric influenza-related deaths since the season started, according to the state health department. The department does not gather data for adult deaths.

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