The O’Neill family: Cara O’Neill, Eliza O’Neill, Beckham O’Neill and Glenn O’Neill. (Anna Tenne Photography)

For two years, there was little the O’Neills could do but watch.

They watched as the body of their daughter Eliza, then 4 years old, grew with each new day, and they watched as her mind slowly reversed. They watched her favorite things — like wrestling with her big brother — fade from interest. She stopped reciting the alphabet. She no longer whispered “I love you.”

The “Twinkle, Twinkle, Little Star” tune she would sing aloud soon became just “star,” then nothing at all. And at home, she would dash past her parents as if they weren’t there, as if she saw right through them. Her personality, just budding, began to erase.

This, they knew, was only the beginning.

Doctors found Eliza’s disease in 2013 when she was 3½. Sanfilippo syndrome is the proper medical term, but its cruelty is best captured in its nickname, “childhood Alzheimer’s.” Like Eliza’s case, children with the rare disease are often misdiagnosed at first because in its earliest stages, Sanfilippo looks like autism.

That’s what doctors first labeled Eliza’s condition three years ago, but the girl’s behavior made her parents, Cara and Glenn O’Neill, feel something more was wrong. An MRI revealed Eliza’s enlarged head. Bones in her neck appeared shorter and flatter than they should be. The toddler was not getting better.

They visited a geneticist, who ordered a special urine test. Next came the terminal diagnosis.

Eliza O’Neill, 6, sits on the lap of her father, Glenn O’Neill. (Anna Tenne Photography)

As a pediatrician, Cara had studied Sanfilippo in school. She knew it was devastating, knew what was to come in the years ahead. In the beginning, Eliza would stop talking and then stop walking. Eventually, she would no longer be able to feed herself. Her memory would vanish, and she would develop seizures and stop sleeping at night. There would be pain. She would suffer.

And then, in her teens, Eliza would die.

“At that point, the bottom falls out of your entire world,” Cara told The Washington Post, “and you have to figure out where you’re going to go from there.”

The O’Neills turned to Google and within weeks found a researcher at Nationwide Children’s Hospital in Ohio — states away from their South Carolina home — working on an experimental gene therapy to cure the disease. Proven successful in mice, the therapy involved injecting a virus — AAV9 — into the bloodstream. The virus would act as a vehicle, carrying inside it a gene Sanfilippo patients lack. Without that gene, sugar molecules build up in the body’s cells and severely damage the brain. This treatment could reverse that process.

The O’Neills had found some hope.

But without more funding, the gene therapy would never make it to the clinical trial phase, and never help Eliza. The researcher had been working for 15 years, aided by funding from parents of other Sanfilippo kids.

“They were the pioneers,” Glenn said, and the pioneers needed a fresh financial push.

With no guarantee that Eliza would be selected for the trial if it got approved, the O’Neills began furiously fundraising. They established the Cure Sanfilippo Foundation and organized 5K runs and parties, a dance-a-thon, silent auction and golf tournament. They sold bracelets and baked goods. But after nearly a year of asking for money, they had raised just $200,000, and Eliza was getting worse.

They turned back to Google, and leaned on the power of video virality. Glenn asked strangers to help him create a clip that would tell their story, their cause and describe their plea. Photographer Benjamin Von Wong raised his hand.

Within a year, they’d made about $2 million.

It was enough to fund the clinical trial, and soon Eliza was selected to participate in a sort of pretrial study. For a year, doctors observed a group of 25 Sanfilippo children, analyzing the disease’s progression so later they would be able to more accurately gauge treatment success.

Even then, there was no guarantee Eliza would get the treatment, and her big brother, Beckham, started asking what was taking so long.

“He keeps asking when we will get the medicine for her,” Glenn told the Huffington Post in 2014. “He even asked at Christmas if Santa would bring it.”

And so in a fit of desperation, Cara and Glenn made a radical decision: They’d place their family of four in quarantine.

Keeping themselves isolated from other humans would give Eliza’s body the best shot at qualifying for the trial. If she somehow contracted AAV9 — the virus vehicle for her missing gene — Eliza’s body would develop an immunity, the therapy would be useless and she would be ineligible for the clinical trial.

Cara quit her job and Glenn worked from home. They home-schooled Beckham, and Eliza’s therapists came to the house wearing masks and gloves. They thought they’d be isolated for just 100 days. Instead, it lasted 726.

For two years, Cara’s mother dropped groceries by their home twice a week, wiping everything down before bringing it in the house. They held at-home dance parties and taught Eliza to hold her breath in their backyard Walmart pool. Beckham video-chatted with his classmates from school. Cara and Glenn kept fundraising.

Cara O’Neill holds her 6-year-old daughter, Eliza. (Anna Tenne Photography)

Their family hiked on unpopulated trails and explored open fields. They went to the beach, just ones without other people. The kids avoided public playgrounds.

“We were the crazy isolated family,” Glenn joked.

They were, as Glenn puts it, “like E.T.”

“Of course,” he added, “it worked out very well.”

In the spring of 2016, the doctors finally called, and on May 10, Eliza, now 6, became the first child in the world to receive the gene therapy treatment. When she left the hospital after a few days of recovery, the O’Neills broke their 2-year quarantine.

Then, Glenn and Cara said, they immediately started thinking about the other Sanfilippo parents.

“These other parents should be feeling what we’re feeling,” Glenn said. “Every moment there is something working against you. Every moment they’re fading away from you, every moment, before your eyes.”

Cara O’Neill with her daughter, Eliza, 6, at the hospital after her gene therapy treatment for Sanfilippo syndrome. (Anna Tenne Photography)

They’ve set a new goal: to raise an additional $4 million for research. And they’ve once again tapped the Internet for support. This week, the O’Neills and Von Wong, the photographer, released another video with Eliza’s recent treatment update. But the rest focuses on raising more money for other children and building support for the parents who, unlike Glenn and Cara, cannot slip into hiding for two years, awaiting a cure.

“We cheer our children on for every little milestone, every gain,” Cara says in the video. “How does a parent respond when it’s going in the other direction?”

For now, the O’Neills emphasize that their future is uncertain. The have no idea whether the gene therapy is working. Medically, that will take time. But as parents, they already see a calmness in Eliza, a spark that had gone missing. They treasure her occasional hugs, and revel in each new word she speaks.

On Thursday, as they turned to leave the house, they prompted her with a question: “Eliza, what do we say?”

Her response, simple but strong: “Open door.”

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