Steven Hawking was diagnosed with amyotrophic lateral sclerosis at 21 years of age, according to Bio. Amyotrophic lateral sclerosis is most commonly referred to as ALS or Lou Gehrig's disease, notes Mayo Clinic.
Mayo Clinic explains that amyotrophic lateral sclerosis is a neurological disease that causes neurons to break down and die. Early symptoms of ALS begin with muscle twitching and weakness, usually in the arms and legs. As the disease progresses, ALS begins to affect the ability to control muscles to move and speak. There is no cure for ALS, and the disease eventually leads to death as it impairs the sufferer's ability to eat and breathe.