Muhammad Ali is being remembered not just for his legendary boxing career and his inspiring public persona, but also for the dignity and grace with which he battled Parkinson's disease over the last three decades of his life.
He was first diagnosed with the degenerative disease in the 1984, three years after he retired from boxing.
Repeated blows to the head during his time in the ring are believed to have led to his later health problems. Ali's physician, Dr. Dennis Cope, spoke about his condition in a "60 Minutes" interview in 1996.
"[Ali] has had a development of what's called Parkinson's syndrome. And from our testing on him, our conclusion has been that that has been due to pugilistic brain syndrome resulting from boxing," Cope told CBS News' Ed Bradley.
"All of our testing has indicated that his cognitive function, his ability to think clearly, to understand what's going on, to really analyze situations hasn't deteriorated at all," Cope added. "His mind is fine."
But the disease took a visible toll on his body. Ali developed a tremor and speech became increasingly difficult.
When he lit the Olympic flame at the start of the 1996 summer games in Atlanta, his hand shook as he held the torch high.
Ali looked increasingly frail in recent public appearances, such as the event in October 2015 when he was honored by Sports Illustrated at The Muhammad Ali Center in his hometown of Louisville, Kentucky, along with former opponents Larry Holmes and George Foreman.
But despite the illness, his wife Lonnie Ali said the three-time world heavyweight champ never felt sorry for himself.
"He's not one who says 'why me?' He's a real champion," she told CBS Phoenix affiliate KPHO last month. "I learn every day from this man: the courage, the strength and the grace that he lives with his illness. For most people, it would put them in bed and put covers over them. They would give up. He does not stop. He continues to live life and that's very important."
Before his death on Friday, Ali was hospitalized with respiratory problems, his condition complicated by advanced Parkinson's.
Here are some questions and answers about Parkinson's disease:
Q: What is Parkinson's?
A: Parkinson's is a neurologic disease that robs people of control over their movements. It typically starts with tremors, and is characterized by slow movement, a shuffling gait, stiff limbs, balance problems and slurred speech.
Q: Who gets it?
A: About 1 million Americans are living with Parkinson's, and an estimated 4 million to 5 million people worldwide, according to the National Parkinson Foundation. It usually appears after age 60, although sometimes it can develop before age 40.
Ali was 42 when he was diagnosed with Parkinson's in 1984. The actor Michael J. Fox was diagnosed when he was just 37.
Q: What causes it?
A: The exact cause isn't known but Parkinson's develops when cells that produce one of the brain's chemical messengers, called dopamine, begin to deteriorate and die. Dopamine transports signals to parts of the brain that control movement. Parkinson's symptoms appear after enough dopamine-producing cells die that there's too little of this neurotransmitter in the brain.
Q: Is there a cure?
A: There is no cure but there are a range of treatments, from medications that affect dopamine levels to a surgically implanted tremor-blocking device. Patients also can benefit from physical and occupational therapy.
Q: What's the prognosis?
A: Symptoms worsen over time, usually slowly. The severity of symptoms, and how quickly they progress, varies widely between patients. In advanced cases, people may be unable to walk or care for themselves. They also can suffer non-motor symptoms, including depression and memory and other cognitive dysfunction.
While Parkinson's itself isn't considered fatal, people can die from complications of the disease.
Q: What complications are of most concern?
A: Lung problems are a risk as muscle weakness impedes the ability to cough and to swallow. While any kind of pneumonia can occur, what's called aspiration pneumonia -- when bits of food or liquid land in the lungs instead of being swallowed properly -- is the leading cause of death among Parkinson's patients, said National Parkinson Foundation medical director Dr. Michael S. Okun.