— -- For nearly two years, more than 700 days, the O'Neill family stayed in isolation in their South Carolina home in an effort to save their daughter Eliza.
Eliza, now 6, was diagnosed with a rare degenerative disease called Sanfilippo Syndrome Type A two years ago.
The disease, sometime's called "child Alzheimer's," is a metabolic disorder that means she lacks an enzyme to break down heparin sulfate, which naturally occurs in cells, causing it to build up over time. This buildup renders cells unable to function properly and can affect everything from sleep to speech to movement.
In the last two years, Eliza's parents, Glenn and Cara O'Neill, have watched their daughter lose the ability to sing her favorite song, recite her alphabet and even tell them "I love you."
"It takes away the brain so very quickly," Glenn O'Neill told ABC News. While there is no cure for Sanfilippo, the family made it their mission to do anything they could to get Eliza treatment for the disease. They started a foundation and raised millions online with the hope that an experimental treatment could help her.
Two years ago, they learned that if she contracted a common childhood virus, she would be excluded from participating in one promising gene therapy trial. In order to protect her, the entire family, including Eliza's older brother Beckham, isolated themselves indoors.
"It's all a very, very tough call to do it," O'Neill told ABC News. He said initially the family was hopeful it would only take a few months of being separated from the community before Eliza was placed in an experimental trial. However, weeks stretched to months to years.
"It went from 100 days to 300 days and 500 days," O'Neill said. The family spent holidays and birthdays alone in the house with some minor contact through windows, doors and special protective wear. "Once we started, we almost couldn't stop."
This spring -- after more than 700 days in isolation -- the family finally got the call they had been waiting for. Eliza would be tested for an experimental genetic treatment.
While the family's foundation did help fund part of the research, along with other Sanfilippo foundations, patients can't pay to take part in experimental research. Nationwide Children's Hospital, where the study was scheduled to run, confirmed to ABC News this was true of their study, as well.
After days of testing, the researchers based at Nationwide Children's Hospital agreed to let Eliza into the test. O'Neill said even after her inclusion, he was still worried that something could happen to make her ineligible.
"After all this time and the difficulty of the journey, you just get paranoid," said O'Neill. "I wanted to just cradle her for a week and hold her in my harms and hug her."
He said that feeling remained right up until she received the genetic therapy, the first of its kind to be used on a child with Sanfilippo syndrome.
"It wasn't until she went into the room and was put under and the doctors came out of 30 minutes later" to say she had received the medication "that we were able to let down and cry and say this is happening," O'Neill said.
But after years of trying to get his daughter help, he immediately felt they needed to help other families going through the same heartache.
The family is hoping that by sharing Eliza's story they can continue to draw attention to the rare condition and raise funds for other treatments. They hope to raise at least $2 million more, for a total of $4 million, to continue Sanfilippo syndrome research through their Cure Sanfilppo Foundation. Nationwide Children's Hospital confirmed that the foundation has helped support their research into the disease.
After Eliza's treatment, the family not only had to wait to see how she would respond, they had to get used to being among people again for the first time in 726 days. O'Neill recalled going to the grocery store again in the days after Eliza received the gene therapy treatment.
"It's a little bit like coming out of Twilight Zone," said O'Neill. "It's taken some adjustment."
His son Beckham is now going to school again, entering fourth grade with other students for the first time since he was in first grade.
But as he watches his daughter, O'Neill said it makes the years spent in isolation worth it.
As the family readjusts to walking among the community again, O'Neill said he's hopeful that the experimental medication will help halt some of the disease symptoms. In his opinion, his daughter has been more responsive and present than before she was on the medication.
"To see her do these things and interact and see these things that we thought were gone forever," he said. "All we want to do is spread this story far and wide."