© FACEBOOKKatharine with her parents Liz and Phil Gallagher
After initially thinking she was recovering, she was found dead on the bathroom floor by her devastated boyfriend of two years, Brendan Carey.
Brendan (44) told MailOnline that he called the paramedics to his Tustin, California, home but it was too late.
Her heartbroken mother, Liz, told Southern California TV news station KTLA that "walking down the aisle behind her casket was probably the worst thing that's ever happened to us."
Katharine fell ill with the usual flu symptoms such as fever, chills, and nausea, and went home from work sick on Friday night. She attempted to "sleep it off", but when her symptoms had not eased by Sunday she attended a local emergency clinic for antibiotics and an IV.
Katharine developed severe acute bronchial pneumonia passed away on Tuesday, December 5th 2017. Her mother confirmed that she did not have the flu shot.
"Young people just think they're invincible, and most of them don't want to pay what it costs now to go to doctors," an emotional Liz told the news station, urging people to act on early symptoms.
"Life is short, and I guess it's trite to say 'Live every day the best way you can,' but nobody ever thinks it will happen to them," she added.
Katharine, a counter-terrorism graduate of Boston University, was the only child of Liz and her husband Phil Gallagher.
An increase in news coverage of flu-related deaths prove that the seasonal disease should not be taken lightly. According to reports, 42 people under the age of 65 have died in California alone this season.
Comment: Taking one's health and well-being seriously is good advice, but there are more effective and safer ways to boost the immune system than getting a flu shot: