Even with rest, her condition worsened, and Thomas was taken to the emergency room.

By Jason Duaine Hahn
Updated February 26, 2021

A mother of three from California, who was an avid runner and enjoyed being active, died within days of showing symptoms of influenza—a virus that has led to the deaths of dozens of people in the state and across the country this season.

Katie Denise Oxley Thomas appeared to be happy and healthy when she rang in the New Year with girlfriends. But just two days later, as Thomas was getting her three children ready to visit their father in San Diego, she came down with a fever and a sore throat. The 40-year-old then visited her doctor, who diagnosed her with influenza and advised her to rest at home.

Even with rest, her condition worsened just 24 hours later, and Thomas was taken to the emergency room at the nearby Good Samaritan Hospital in San Jose, California. Once again, physicians recommended she return home to recover, but later that night, Thomas struggled to breathe, and she was driven back to the hospital by her boyfriend. Doctors scanned her lungs and discovered she had pneumonia and they quickly placed her on a ventilator. Early the next morning, she went into septic shock.

“It was very, very fast,” Thomas’s sister, Amber Oxley, 34, a fashion designer from Brooklyn, tells PEOPLE. “My brother went straight to the hospital and spoke to the doctor, and they told him they should put her kids on a flight and bring them in. That was a real wake up call that she may not survive. None of us expected to hear that at all.”

More than 200,000 Americans are hospitalized each year because of the flu, leading to some 36,000 deaths. The virus because increasing deadly when it leads to pneumonia, as it sometimes does, and according to the CDC, the flu and pneumonia combination was the eighth leading cause of death in 2016. The center adds that each year, more than 1.5 million people in the United States experience sepsis, which causes more than 250,000 deaths.

A team of doctors was called to the Intensive Care Unit to help stabilize her fever as it continued to rise on Thursday morning, but Thomas soon lost consciousness.

By sunset on January 4, Thomas was pronounced dead, just two days after her initial flu diagnosis.

“Katie was very energetic, full of life, and was always the life of the party. She had so much energy, she just turned 40 and was very fit, just loved running, and had recently gotten into yoga and pilates,” Oxley says. “She was an extremely healthy person and there weren’t any warning signs at all. She was a really awesome mom, and loved her kids to pieces. ”

Oxley says the family isn’t sure if Thomas received a flu vaccination or not, and an autopsy this week will help determine if she did. Thomas’s death comes during a flu season that has been particularly deadly, and California has seen at least 27 people younger than 65 die from the virus since October. The CDC says that even though the flu shot isn’t 100 percent effective, it can still lessen the chance of someone catching it by 10 to 60 percent.

The family does not hold any “hard feelings” toward the doctors who initially saw Thomas, Oxley says, and she hopes people who experience flu-like symptoms push to receive more tests from doctors to determine what strain of the flu they have and to check for pneumonia.

“I’m just sad, I’m just really, really sad,” Oxley says. “I’m a paranoid mess. It’s terrifying that something so contagious can be so dangerous. I feel like I’m still in shock. It feels so crazy that she could die so quickly. We’re really sad, and we’re really going to miss her.”

This Story Originally Appeared On People