Emily Whitehead Foundation
It was 2010, and 5-year-old Emily Whitehead had her entire future ahead of her.
"From the time she was little, she liked to pull pranks and make us laugh," her father Tom, 53, tells PEOPLE. "She made our lives a lot better."
The rambunctious preschooler had just gone to her annual checkup in Philipsburg, Pennsylvania, hit every milestone, and was given a clean bill of health.
"Everything was great," recalls her mom, Kari, 46. But just a week later, she noticed that Emily had bruises on odd parts of her body, including her stomach and back. Then her gums started bleeding, and she was waking up in the middle of the night with unbearable pain.
When Kari Googled the symptoms, "they were the classic signs of leukemia," she says, and by the next morning she was back at the doctor's office with her little girl.
Within days, Emily was diagnosed with acute lymphoblastic leukemia and doctors at Penn State Health in Hershey, Pennsylvania, prescribed a 26-month regime of chemotherapy for Emily.
During the first few weeks of treatment, she ran dangerously high fevers, and had a rare infection that almost caused doctors to amputate both her legs. But despite the challenges, she was in remission one month later.
"We had a rough start," says Kari, "but the doctors said when chemotherapy works for these kids, it works."
That was until October 2011, when Emily relapsed and the then 6-year-old was given just a 30% chance of survival.
"The news was more devastating to us than her original diagnosis," says Tom, a power company lead lineman, who also took Emily to get a second opinion at Children's Hospital of Philadelphia (CHOP). "I told Emily that if I had to crawl to the North Pole, I would, if that's what it took to find someone to fix her."
And essentially, that's exactly what he and Kari, a research project coordinator, did.
As Emily spent most of the next four months in the hospital preparing for a bone marrow transplant in February 2012, her parents made calls to experts and found out about every possible treatment available.
"I was just praying like, 'God, if you're up there, we need help right now.' I was kind of asleep but not really and I suddenly saw Emily at CHOP. And I could see her getting better," says Tom, who wrote about the clear visions he had of Emily's recovery in his book, Praying for Emily: The Faith, Science, and Miracles that Saved Our Daughter.
From that vision, he knew, he says, that she would be alright.
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Emily Whitehead Foundation
"I know my parents tried to make me smile every day," Emily, now 17, tells PEOPLE. "That's something really special that I still remember."
By February 29, her condition had deteriorated to the point that she was no longer eligible for a transplant — and the family was out of options.
But as fate and science would have it, Tom and Kari got the news from Emily's doctors at CHOP that after a year of completing the required paperwork, the FDA and other committees had finally approved the hospital's Phase 1 clinical trial for CAR T-Cell therapy in children — and Emily had become the very first pediatric patient.
There were many risks involved, but "it wasn't a hard decision for us at all," says Kari. Adds Tom: "The alternative was to go home on hospice and just watch her die."
And miraculously, it worked.
On May 10, 2012, 23 days after Emily began the treatment, a bone marrow test showed all of her cancer was gone.
"It was a total shock after everything we she'd been through," remembers Kari, whose family is featured in the new film Of Medicine and Miracles that premiered at the Tribeca Festival in June. "We were just so excited."
For the full story on Emily Whitehead's miraculous survival and recovery, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here.
Children's Hospital of Philadelphia
Dr. Stephan Grupp, director of the Susan S. and Stephen P. Kelly Center for Cancer Immunotherapy and Emily's doctor who is also in the documentary, says that "she was not expected to make it and then suddenly all of the cancer was gone."
There are a couple different approaches "to getting the body's immune system to directly engage and kill the cancer, but CAR T-cell therapy is the most powerful and direct way," he adds.
The treatment involves taking T-Cells — a type of white blood cell critical in fighting infections — from the body, genetically engineering them in a lab over a three-week period to "teach them how to fight the cancer," says Grupp, then putting them back in the patient's blood.
Since Emily became the first pediatric CAR T-cell patient, more than 15,000 people with blood cancer have successfully received the treatment worldwide.
"You could argue this is a brand-new field of medicine," says Grupp. "Now, we just have to find the right recipe to treat all types of cancer."
Ever since, Emily, who recently got her driver's license and is applying to colleges, has remained cancer free and when she turned 17 in May, was declared cured.
In 2015, she and her family started the Emily Whitehead Foundation, to raise awareness about innovative childhood cancer treatments and help other families dealing with the disease.
"Spreading awareness about treatments like CAR T-cell is really important to me," says Emily. "It's a miracle I'm alive — and I am so grateful."
For more on the Whitehead family, and Emily's groundbreaking cancer treatment, pick up a copy of PEOPLE, on newsstands Friday