Zach Parnaby was born a healthy baby boy in Northeastern, England.
As he grew, the toddler's mother Lindsey said he loved building with blocks, going for walks and feeding deer at the park.
"Zach was mischievous, cheeky, and extremely brilliant," she said. "He would walk and hold onto our hands, or the couches. He was a normal developing child up until last year. Then, over a couple of days he just stopped walking."
Parnaby, 29, and her husband Ben, 29, immediately grew concerned.
At first, Zach's regression was thought to be a phase and doctors were confident that it would subside. But by early December, the now 20-month-old's motor skills had begun to shut down.
"He [Zach] started sitting in a different position and by the end, he was sitting and falling back." Parnaby told ABC News. "He started losing all ability to do things, so we insisted on not leaving the hospital."
On Dec. 17, Zach's doctors were confident that he had some form of cerebral palsy.
"They said it was one of two forms, both were terminal," Parnaby said. "They couldn't tell us which one it was until they did more tests. I didn't want to believe it.
"By this time he wasn't even able to sit up. He was quite disabled."
The hospital would not comment on the case when reached by ABC News, saying that privacy laws prohibit them from releasing patient information.
On Christmas Eve, the Parnabys' said their worst fears were confirmed. Zach was diagnosed with Krabbe leukodystrophy, a rare genetic disorder that affects the nervous system.
Doctors told the couple that their son would have just two years left to live.
"I think we were numb, to be honest," Parnaby recalled. "You hear of children being diagnosed, but you never expect it to happen to your own."
With the heartbreaking revelation of Zach's life being cut so short, his mom and dad decided that they were going to make every single day count.
About one month after his diagnosis, the Parnabys created a bucket list for their son.
"We couldn't sit home and feel sorry for ourselves and be morbid every day because in turn, it would make Zach feel that way," Parnaby said. "We took the stance that if Zach is smiling every day, then that's what matters.
Many of the 16 items on the list are things that Parnaby said, her and her husband would've wanted to do over the course of Zach's life like riding a bike, meeting Santa, seeing a waterfall and more.
Six of the wishes have already been completed including owning his own dog, and getting to meet a famous person.
"He got to meet the Newcastle United team," Parnaby said. "His dad would’ve taken Zach to many matches as he grew up."
To aid in Zach's medical care and to help fulfill his bucket list wishes, Zach's family has created a GoFundMe page titled "A Promise For Zach."
In the future, Parnaby said she'd like to use any remaining donations to help families with terminally ill children.
"When we experienced that diagnosis, we were sent away with some facts and medication," she said. "I had to go out and find support. We want to help families and help them over the crisis, to fill that gap and be there for them.
"Prior to the onset of this disease Zach very rarely cried and he was just genuinely happy; very happy. It's keeping us focused and when we're out there and doing things he's much more relaxed.
"Other families can do what we're doing and make that child's life as happy as they can in the short time they have."
Zach's Bucket ListFind a cure for Krabbes disease For Krabbes disease to be screened for at birth To meet Mickey Mouse - completed. To ride in a fire engine and meet Fireman Sam - completed. To swim in the sea To build a snowman - completed. Go sledging Meet a real bear (I love bear's and this was one of my first words) To meet Santa To ride a bike Meet a famous person - completed. Ride on a red London bus Take a boat trip - completed. Own my own dog - completed.(SAM COMES HOME SOON) See a waterfall and swim near it if it's safe Take a ride on Thomas The Tank Engine - booked for March 22.