Stephen Hawking’s rare neurodegenerative disease defined him almost as much as his physics. He used his celebrity to put the spotlight on muscle-wasting amyotrophic lateral sclerosis, posting videos of his family taking the ice-bucket challenge on his behalf to raise funds and awareness about a disease that often draws sympathy but rarely money for drug development and research.
The definition of what constitutes a rare disease varies widely, with the US defining it as a condition that affects fewer than 1 in 200,000, and the European Union using it to describe