ALS is a nasty neurological ailment without a cure to date that affects about 12,000 to 15,000 Americans; it eats away at the body’s ability to voluntarily move muscles, eventually sapping essential functions like walking, talking, eating, and breathing. Hawking was diagnosed with the degenerative disorder in 1963, when he was just a 21-year-old graduate student. He was given less than three years to live by doctors.

And yet, live he did, for another 55 years and against all odds (though he had to live a substantial part of his life in a wheel chair and speaking with the assistance of a computer). “Most people with ALS die from respiratory failure, usually within 3 to 5 years from when the symptoms first appear,” according to the National Institutes of Health (NIH). Just about 10% of patients live for 10 years or more.

So Hawking is an extreme outlier in this regard. And the precise reason he was able to live with the disease for so long is unclear. “This is fairly untypical,” says Lucie Bruijn, chief scientist at the ALS Association.

The medical mystery underscores how much work is left to be done—and how much remains unknown—in the quest to treat and possibly cure ALS. But there’s also been some incremental progress in the field in recent years. In 2017, the Food and Drug Administration (FDA) approved the first new drug to treat ALS since 1995. (Though it’s important to note that the treatment, Mitsubishi Tanabe Pharma America’s Radicava, is mightily expensive and only treats symptom progression rather than the root cause of the disease.)

Scientists have also made recent breakthroughs into understanding the genetic causes of Lou Gehrig’s. About two years ago, funding from the “Ice Bucket Challenge”—the viral video trend that raised millions for ALS research and was initially met with a big dollop of skepticism—helped researchers identify a gene associated with the disease.

This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.