"We are just trying to save her life," Glenn, 42, tells PEOPLE

On Tuesday, Glenn O’Neill crossed off the number 453 from a large white sheet of paper hanging in the kitchen of his Columbus, South Carolina, home.

It’s the 453rd reminder of how many days he and his family have been living in self-imposed quarantine. “We are just trying to save her life,” Glenn, 42, tells PEOPLE of his daughter, Eliza. “The piece of paper reminds us how far we have come and how much further we need to get it.”

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Eliza O’Neill, a bubbly 5-year-old, was diagnosed in July, 2013, with Sanfilippo syndrome, type A – the most severe form of the rare genetic disorder. It’s causing her to lose the ability to speak, and by her next birthday, she might not be able to talk, eat or walk on her own. By 10 years old, most children with the disease are wheelchair bound and need around-the-clock care.

“We have to try very hard not to think about what will happen,” says her mom, Cara O’Neill, 40. “We have hope she might end up being okay.”

Eliza has one shot at surviving – a clinical drug trial. Over two years, the family raised the $2.5 million needed to get the drug production started. Through a GoFundMe page, as well as traditional fundraising, the family met their goal last December. But what came next has proven to be even more difficult than raising the large sum of money.

Protecting Eliza from Common Germs

The family has chosen to live in isolation to prevent Eliza from getting sicker – even the common cold can accelerate her symptoms. It’s also crucial that she doesn’t develop antibodies to a particular, common virus that could eliminate her from the study.

“Her body would then reject the gene therapy,” Glenn told PEOPLE in July. “So when the trial happens, she wouldn’t be able to take part in it. We are not going to risk that.”

So the rare times that visitors come over to visit Eliza, Cara, Glenn and their 8-year-old son Beckham, they have to wear a mask and gloves.

New Family Games and Activities

Inside the home that has become their safe haven, the O’Neill’s are creative and playful. Eliza, who is starting to forget words she always knew and can’t easily put sentences together, asks her parents to show her pictures of different animals on the Internet.

“She understands what Google is,” Glenn says. “It’s a fun exercise to keep her practicing different words.”

On Monday, the family went out to their backyard and acted out a script that Beckham wrote.

“We do a lot of things that regular families do,” says Glenn. “We just have so much more time at home.”

Only Rare Trips Away from the House

Although Beckham is a trooper and understands that his family has gone to extreme measures to try and save his sisters life, he can’t wait to visit Lego Land – and is even excited to walk down the aisle of a grocery store.

As they wait for the drug trial to begin, the family only occasionally goes outside, and only to isolated locations where they don’t interact with anyone else.

“Last week we went to this state park and had a picnic on this bridge that Eliza calls ‘troll bridges’ because that’s what they call it on Dora the Explorer,” Glenn says.