The parents of 7-year-old Adalyn Graviss are mourning the loss of their unvaccinated daughter, who died less than 72 hours after testing positive for COVID-19.

Adalyn, a second-grader from Knoxville, Tennessee, died on Feb. 7 after developing a severe neurological response to the virus.

"She was just a happy, healthy, normal, beautiful soul," her mom, Jennifer, told Good Morning America. "She was just so sweet, an amazing kid."

Adalyn, complained of feeling hot and they took her temperature, which was 102, and they had her take an at-home COVID-19 test that came back positive.

Adalyn stayed home from school that day and was well enough to play in the front yard, when "all of a sudden" she became severely sick, Jennifer told Knox News, and was struggling to walk or speak.  

"It was right around the nine o'clock hour when we noticed her speech was all but gone, though she was still responding to us," her father Adam told GMA. "By 10 o'clock, I was in the emergency room [with her], and she was unresponsive at that point."

"It was just so fast," he added. "Hours before going to the hospital, she was running in the front

Doctors speculate that her severe reaction to COVID-19 could possibly be connected to her Raynaud's syndrome, a common condition in which parts of the body feel numb in response to cold temperatures and stress.

But Adalyn's sudden death "is a big reason of why we don't roll the dice on a virus like this," one of her doctors, pediatric infectious disease expert Dr. Isaac Thomsen, told GMA. "This is not something to mess around with."

"The takeaway for parents is this is a virus that we have got to take very seriously and one we have a safe and effective vaccine for," he added.

Jennifer and Adam had been considering taking Adalyn to get her COVID-19 vaccine after she became eligible in November, but held off and continued to follow safety precautions like wearing masks and washing their hands. Adalyn was particularly diligent about wearing her mask at school for her little sister Ella, who had been born just a few days earlier on Jan. 28.