Stephen Hawking, who died March 14 at age 76, was a renowned British theoretical physicist and author. He was also an astronomer, mathematician, cosmologist and was considered by many to be the world's greatest living scientist. Hawking also happened to have ALS (amyotrophic lateral sclerosis), a neurodegenerative disorder known as Lou Gehrig's disease.

The disease is merely a postscript on Hawking's life story. Although it eventually robbed him of his ability to move or speak without technological assistance, ALS never defined the man. However, it did bring attention to the disorder and Hawking's determination to overcome any obstacles it threw in his path.

"I am quite often asked: how do you feel about having ALS?" he once wrote. "The answer is, not a lot. I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many."

What is ALS?

ALS is actually a group of rare neurological diseases that involve the nerve cells that control voluntary muscle movements like talking, walking and chewing, according to the National Institutes of Health.

ALS is part of a larger group of disorders known as motor neuron diseases, which are caused by gradual deterioration and death of motor neurons. Those are the nerve cells that go from the brain to the spinal cord and to the body's muscles. These motor neurons are key for communication between the brain and voluntary muscles.

With ALS, the nerve cells lose their ability to send messages from the brain to the muscles. The muscles gradually weaken and lose their ability to work, so ALS patients first lose their strength, then their ability to speak, eat, move and breathe.

Men are slightly more likely to develop ALS than women. Although it can start at any age, it most commonly develops between the ages of 55 and 75.

The disease is progressive and there's no cure or treatment to keep symptoms from getting worse. Most people die from respiratory failure, usually within three to five years after symptoms first appear. Only about 10 percent of people who have ALS survive a decade or more.

No one knows for certain what causes the disease, however evidence suggests that genetics and environment play a role.

Hawking's ALS journey

Hawking was diagnosed with ALS in 1963, shortly after his 21st birthday. By the end of the '60s, he had lost mobility to the point that he was using a wheelchair. By the late '70s, his speech had begun to deteriorate. After an attack of pneumonia and a tracheotomy when he was 44, he lost the remainder of his ability to speak. From then on, he communicated with the help of a computer synthesizer attached to his wheelchair.

Hawking lived with ALS for more than 50 years, an incredible lifespan for someone with the condition. Hawking is thought to be one of the disease's longest survivors, according to the Rare Disease Report. Many believe that his longevity could be linked to his genetic makeup or his early diagnosis, both of which have been linked with increased survival.

Triumph over adversity

Like so many who face serious illness, when Hawking was first diagnosed, he felt sorry for himself.

"I felt it was very unfair — why should this happen to me?" he wrote in his 2013 memoir, "My Brief History."

"At the time, I thought my life was over and that I would never realise the potential I felt I had."

He said he didn't want to ask doctors for details, because he knew the diagnosis was grim. So he kept learning and advancing to the forefront of his field, taking any necessary steps that would help him move, eat or be understood, but never letting the disease stop him.

In 2014, when the Ice Bucket Challenge swept social media, raising more than $220 million for ALS research, Hawking couldn't take part because the risk of pneumonia would be too great, but he offered a video of his children — Robert, Lucy and Tim — "gallantly" participating on his behalf.

Professor James Hartle, who worked with Hawking to create the Hartle-Hawking wavefunction to explain the Big Bang, told BBC Radio Four, "My memory of him would be … first our work together as scientists and, second, as a human being whose whole story is a triumph over adversity [and] who inspired a lot of people, including me."

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

How Stephen Hawking brought attention to ALS
Stephen Hawking, the renowned theoretical physicist, lived for 50 years with ALS, a group of rare neurological diseases, but he never let it define him.