A 27-year-old woman died from complications from the flu just days after she contracted the virus that has taken the lives of dozens of people, young and old, across the U.S. this season.

Katharine Gallagher, of Tustin, California, left work early on Friday, December 1, when she developed a fever, diarrhea, cold sweats, body aches and nausea. Five days later, she was dead.

Her mother, Liz Gallagher, says her daughter thought she was just run down and could recover on her own with a little bit of rest that Friday.

"I asked her if she would like to go to the doctor, and, as a typical young person, she said she was fine and that she was just exhausted, and needed sleep," Liz Gallagher tells PEOPLE. "She felt the same way on Saturday when I talked to her, but by then, she really had an issue with nausea, dizziness and coughing."

Katharine — a graduate of Boston University who worked as a server at a local brewery — was taken to urgent care by her mother that same day. Though the facility didn't take Katharine's insurance, Liz felt deeply that her daughter needed care right then, so they stayed, and physicians prescribed antibiotics and gave her fluids through an IV to help with her dehydration.

Katharine felt better the next day, despite her coughing and back and stomach ache, but by Tuesday, her dizziness had worsened and she was having difficulties breathing.

"I wanted her to go to my doctor because she had never been to her doctor since she had never really been sick. We chatted by text several times that day, and it even got a little heated," Liz recalls. "She's writing back in capital letters telling me that I should just let her take the medicine, get sleep and she'd be better. She wanted to give the antibiotic a chance to work."

Katherine sent her mother a text around 3:20pm saying she managed to lay down, and Liz wrote back to encourage her daughter to get sleep, and to contact her once she was awake. But, less than three hours later, the next call came from Katharine's boyfriend, who found her unconscious and slouched over in their bathroom. He started chest compressions while waiting for paramedics to arrive.

"After about 10 minutes, he said to me, 'They've called it,' " Liz says, "It was a total and complete shock."

Katharine was pronounced dead on December 5, about three weeks before her 28th birthday and just days after she initially showed symptoms of the flu. Katharine had severe acute bronchial pneumonia, a complication of the virus.

The Centers For Disease Control says that even though the flu shot is not 100 percent effective, it can still lessen the chance that someone catches it by 10 to 60 percent. Katharine did not receive the flu shot before experiencing symptoms, Liz says.