Even the day 15-year-old Matt Scozzari learned his girlfriend was dying from a disease that had killed her mother and grandmother, he knew he wanted to stay with Sabrina Parker for as long as she lived.

He’d already sensed that something was gravely wrong with the girl who had stolen his heart. She was having trouble breathing and slurred her words. She’d lost a lot of weight. But when he asked what was going on, she brushed it off.

Then, at Sabrina’s “Twilight”-themed 16th birthday party, a couple of her friends pulled him aside.

“Matt, it’s about Sabrina,” one said, according to the Associated Press. “She’s got Lou Gehrig’s disease.”

Though he wasn’t that familiar with the illness, he knew it was very serious. Feeling as though he was going to faint, he walked in a fog to a pond on the property, sank down on the grass and sobbed.

That’s where Sabrina found him.

“Are you going to leave me now?” she asked, according to the AP.

“I will never leave you,” he told her. “No matter what.”

They clung to each other and wept.

Only a month after they began dating, Sabrina learned the horrible news: She had amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease because legendary New York Yankee Lou Gehrig was diagnosed with it in 1939. The debilitating and ultimately fatal condition is caused by the degeneration of the motor neurons – the nerve cells that control muscle movement.

The high school freshman had been dreading the day she might be told she had the illness, since both her mom and grandma had died of it. Once doctors confirmed what Sabrina had long feared would happen, she’d been able to let her best friends in on her secret. But she couldn’t muster up the courage to tell Matt.

Sharon Matland, the ALS Association’s vice president of patient services, said it’s unusual for someone so young to have the illness. The onset of the disease typically occurs between the ages of 40 and 70, and most who develop ALS only live for two to five years after it starts.

“It’s pretty rare to be diagnosed with it [at her age], but it’s familial,” Matland told AOL Health.

Patients can experience a range of side effects, from respiratory problems to muscle weakness in the arms and legs and a tendency to trip or fall.

“Symptoms can vary,” said Matland. “It’s a very complicated disease.”

Only about 10 percent of ALS cases are genetic, but its cause remains largely a mystery. Highly trained athletes and those in the military are particularly susceptible.

“It’s difficult to know what causes it and what precipitates it,” Matland said. “It could be something environmental, but we just don’t know.”

Until the illness began to overtake her, Sabrina had been, in many ways, a typical teenager. She was a regular on Facebook. She was obsessed with the “Twilight” movies and one of the heartthrobs who stars in them, Taylor Lautner. She loved roller skating, volleyball and softball and had a soft spot for Taco Bell food.

Her maternal grandmother, Lorna Melton, died of the disease at age 46, the year before Sabrina was born. When she was 4, the illness also took the life of her mom, Missy Melton. She was only 24.

Sabrina went to live with her paternal grandparents a couple of years later. They were terrified that the little girl would succumb to the same fate. For a while, though, things seemed hopeful.

But last year, the first signs appeared. Sabrina began suffering from earaches and her voice changed to become more nasal and less easy to understand, the AP said. During gym class, she couldn’t raise her head up off the floor while she was doing sit-ups.

She went to numerous doctors and had scores of tests before she was diagnosed with the terrible disease that was slowly draining her of life. She and her family were devastated.

Matt hadn’t had an easy time growing up, either. A mild learning disability during elementary school left his parents and doctors thinking he was autistic and caused him to have problems speaking and reading. The other kids made fun of him mercilessly. Maybe that’s why he was so patient and understanding with Sabrina, why he related to her so much, why he loved her so deeply.

After he learned the truth, Matt remained devoted – even when Sabrina had to get a feeding tube and lost her ability to speak clearly. He stayed with her constantly, through surgery after surgery and several bouts of pneumonia. He became her translator and listened to all her hopes, dreams and fears. They invented their own sign language.

Matt wanted to marry Sabrina, but his parents knew she wouldn’t be alive much longer. So they suggested the pair have a friendship ceremony instead. It was held before the people closest to them on November 20. Those who witnessed it clapped, and cried.

Nine days later, Sabrina’s grandparents knew the end was near. They called Matt. He held her hand through the night. Sabrina died in her sleep, with the boy who loved her with all his heart by her side.

Before her body was taken away, Matt kissed her gently on the head.

“Thank you for saving me,” he told her.