For the last seven years, a rare neurological disorder ravaged Mitchell Herndon's body.

As the condition — a genetic mutation diagnosed in only a few people in the world — robbed the Missouri teen of his ability to walk, took his hearing and then his eyesight, Herndon made a decision: If the disease killed him, he would donate his body to science in the hopes of saving others.

Last Wednesday, just days before a potentially life-saving drug would have been made available to him, Herndon, 19, died. Abiding by his wishes, Herndon's family chose to gift his body to Washington University in St. Louis for research into neuro-muscular diseases — something that doctors say will be invaluable for advancing the understanding of more than his own disorder.

"It's an incredible tool which he has donated. This will have an impact for many people that get identified with his condition in the future, as well as other people with other neurodegenerative conditions" such as amyotrophic lateral sclerosis (ALS), or possibly Alzheimer's and Parkinson's diseases, said Dr. Bob Bucelli, the neurologist who treated Herndon for the past year and an associate professor of neurology at Washington University School of Medicine in St. Louis. "It's a limitless resource that he's given and incredible what he's offered the medical community by doing that."

In May 2019, Herndon was the subject of an NBC News Digital documentary viewed 4 million times about what it was like to be living with a mysterious disease that kept progressing as doctors raced to try to save him.

Herndon, of Affton, Missouri, had been a healthy, athletic child when he started experiencing difficulty moving his legs at age 12. He was eventually diagnosed with a rare mutation of the ACOX1 gene, which until recently had only been diagnosed in one other person: a teenage girl in South Korea who is unable to communicate. Because the condition is so uncommon, it does not yet have a name.

Over the years, Herndon was in and out of the hospital. He lost the ability to walk multiple times, gaining it back to some degree thanks to physical therapy and medications until a relapse last fall left him in a wheelchair.

While many patients with extraordinarily rare diseases now find others like themselves thanks to genetic testing being cheaper and more widely available, Herndon never did. He found some companionship in the deaf community and among others with muscular disorders, but told NBC News in May that he would have loved to meet someone who could relate to the ups and downs of his particular disease.

“If I knew someone who was 50 years old and had the same thing, if they were doing amazing, that would clear up a lot of anxiety."

“If I knew someone who was 50 years old and had the same thing, if they were doing amazing, that would clear up a lot of anxiety,” Herndon said at the time. “If we found out this is progressive, that would suck, but at least I would know what to expect.”

Mitchell Herndon in 2018. Courtesy of the Herndon Family

Despite the unique challenges Herndon faced, he kept a positive outlook — often using humor to lighten the mood during his lengthy hospital stays, spending as much time as he could with his siblings, Maxwell, 17, and Miranda, 11, and — when he was well enough — attending St. Louis University where he enjoyed studying political science and theology.

Researchers knew Herndon's condition was going to get worse, but they were not sure how quickly he might decline. Then they stumbled upon something that they believed could stop the progression and possibly save his life.

Dr. Hugo Bellen, an investigator with the Howard Hughes Medical Institute and a professor at Baylor College of Medicine who studies genetics and neurobiology, was studying Herndon's mutation in fruit flies. Bellen discovered that a powerful antioxidant, NAC-Amide, showed promising results in stopping the disease's decline. But the medication was not approved by the Food and Drug Administration for use in patients yet.

Bucelli, Herndon's neurologist, worked tirelessly with the FDA to establish a protocol for the medication that would have been considered safe to try on Herndon. As Herndon worsened during his most recent hospital stay, eventually becoming unresponsive and going on life support, the FDA finally granted approval for Herndon to try it— barely an hour after an MRI showed the disease had spread to his brain.

With irreversible brain damage, it was too late to try the drug. Herndon’s family made the painful decision to remove him from life support — something Herndon had expressed to them that he wanted should he ever get to that point. The following day, held by his father, mother and brother, Herndon died, his mother, Michele Herndon, said.

Hope for a young patient with the same condition

While the drug approval came too late for Herndon, doctors have identified another patient who appears to have the same type of mutation as him: a young child in Ohio. Bucelli said he is sending the research he did on Herndon to the physicians in Ohio, which should open the door for them be able to receive the drug for their patient.

"Mitchell could potentially have a direct impact on this next patient."

"Mitchell could potentially have a direct impact on this next patient," Bucelli said.

In the meantime, Bellen has submitted the first-ever paper on Herndon's particular mutation for publication in a scientific journal. He said he suspects there are more patients who will be discovered to have the condition, and in the paper, Bellen proposed a name for it: Mitchell Disease.

Herndon's parents said their son was always eager to help medical professionals, whether it was letting students practice taking medical histories on him or allowing newer nurses to do procedures on him, even if there were more experienced nurses available. His mother said she hoped helping to find cures for this condition as well as others will be part of her son's legacy.

"Our decision to donate his body was just another way that we know he will continue to advance medical research and hopefully pave the way for future patients with his genetic mutation," Michele Herndon told NBC News via email. "We always knew that he believed his body was just that — a body. And his soul is what would live on."