What is Huntington's disease? From the UC Davis HD Clinic: http://www.ucdmc.ucdavis.edu/huntingtons/about.html

Huntington’s disease (often referred to as HD) is a genetically inherited, degenerative, neuropsychiatric disease – a brain disorder. The disease causes neurons to die in the brain, which then causes changes in a person's movement, thinking, and mood. 

HD affects both men and women throughout the world, occurring at a rate of about 1 in every 10,000 people. Onset generally occurs between 30 and 50 years of age, but it varies among individuals and within families, and both earlier and later onset can occur. 

Children of a parent with HD have a 50 percent chance of inheriting the gene that ultimately results in the disease. Inheritance is determined at conception when the genetic ‘code’ either is or is not transferred from a parent who has the HD gene expansion. A person who inherits the HD gene expansion will develop symptoms of HD at some time during his or her life.