I didn't either.
Huntington's disease is a progressive, degenerative disease that causes certain nerve cells in your brain to waste away. As a result, you may experience uncontrolled movements, emotional disturbances and mental deterioration.
Huntington's disease is an inherited disease. Signs and symptoms usually develop in middle age. Younger people with Huntington's disease often have a more severe case, and their symptoms may progress more quickly. Rarely, children may develop Huntington's disease. - Mayo Clinic
What are the chances of someone having it? In the United States alone, about 30,000 people have Huntington's disease; estimates of its prevalence are about 1 in every 10,000 people. (citing) It's not widely known, but it can be highly inhibiting to those with the disorder.