(CBS) -- "You just don't think that a healthy 40-year-old woman is gonna die from the flu." That's what Walt Oxley says happened to his daughter, Katie Oxley Thomas. He says the California mother of three was in excellent health. She practiced yoga and ran three marathons.
"She was just in the peak of her life," she says.
Oxley says Thomas visited the hospital twice in two days – both times diagnosed with the flu and sent home to rest. She was admitted to Intensive Care just days later and died within hours.
"The flu had gone to pneumonia that had gone to septic shock, and it consumed her so quickly it was – it's still hard to grasp," Oxley says.
Thomas is one of 42 people in California under 65 who have died from the flu this season. Nationwide, 26 states are reporting high "influenza-like-illness activity", according to the Centers for Disease Control and Prevention.
"Unfortunately, this year's influenza season is proving particularly difficult," says Dr. Anne Schuchat, Principal Deputy Director of the CDC.
Over the weekend, 10-year-old Nico Mallozzi was traveling with his hockey team in western New York when he was diagnosed with the flu. He died on his way back home to Connecticut.
At least 22 children have been reportedly killed by the flu this season. The CDC's Daniel Jernigan believes that number is likely much higher.
"These deaths are likely under-reported – sometimes maybe half of what's actually out there or even a third of what's out there," he says.
The best way to protect yourself is to get a flu shot. If you haven't, it's not too late. And if you suspect you already have the flu, you should see a doctor.They can prescribe anti-viral medications which can help reduce the severity and duration of the flu.