An ALS patient's life expectancy averages just two to five years after diagnosis, and only about 5% make it past 20 years, the ALS Association notes. But Stephen Hawking, the 73-year-old theoretical physicist and subject of an Academy Award-winning biopic, has beaten the odds and lived with his motor neuron disease for 52 years—and no one's sure why, though some experts suggest factors that may have increased his life span in today's Washington Post. Scientists also don't know for certain what causes ALS (aka Lou Gehrig's disease) in general and are studying genetic and environmental links, the CDC notes. But although the underlying reasons for Hawking's long-term survival are unclear, researchers' thoughts on it aren't. "He is exceptional," neurology professor Nigel Leigh told the British Medical Journal in 2002. "I am not aware of anyone else who has survived with [ALS] as long."
What also makes Hawking's case unusual is that he was informed of his illness so young: The average age for diagnosis is 55, the Post notes, though Hawking's early onset may have helped him in the long run. "Survival in younger patients is strikingly better and is measured in many years—in some cases, more than 10," Leigh previously noted to the BMJ. Leo McCluskey, medical director of the Penn ALS Center, told Scientific American in 2012 that a patient could survive ALS for a long time if he didn't succumb to respiratory failure or deterioration of the swallowing muscles. Meanwhile, Hawking told the New York Times in 2011 that he's been lucky to have top-notch medical care and a job that engages his mind, and that perhaps he just has a rare form of motor neuron disease. "Maybe my variety [of ALS] is due to bad absorption of vitamins," he says, per the Post. (Hawking says for humankind to survive, we have to be nicer.)