"Chorea" comes from the Greek word for "dance" and refers to the uncontrolled movements that mark the HD patient. The illness has been traced back at least as far as the Middle Ages, but was named by George Huntington, an American doctor, in 1872. The illness also has been called chronic progressive chorea, to signify that symptoms become worse over time.

HD causes certain brain cells called neurons to die, eventually affecting the way a person feels, thinks, and moves. The death of the neurons causes jerky movements (chorea) and dementia.

A genetic defect causes production of an abnormal version of a protein called huntingtin. This protein affects neurons in parts of the brain that control movement and thought, destroying them. Researchers are still trying to learn how the abnormal huntingtin protein interferes with normal cell activity.

HD most often strikes in midlife, but it can also show up in children and in older adults. About 15,000 people in the United States have HD, and 150,000 have a 50 percent risk of developing it. HD cases are found worldwide.

A single abnormal gene on chromosome number 4 causes HD. Chromosome number 4 is one of the 22 chromosomes that aren't linked to gender, which means both men and women have equal chances of inheriting HD (called autosomal inheritance). If a child does not inherit the HD gene, the child will not develop HD and cannot pass the gene on to later generations. If the child inherits the gene, that child will eventually develop HD. If one child in the family inherits the gene, it does not make it more or less likely that other children in the family will inherit the gene.

Other early symptoms include difficulty driving, remembering facts or making decisions, and uncontrolled movements of the feet, hands, face or trunk. The earlier the symptoms appear in a person's life, the faster the disease tends to progress. As the disease progresses, the person finds it more and more difficult to concentrate. Emotional disturbances such as anger or depression may ease or may become more pronounced. Uncontrolled movements become worse, and coordination and balance are affected. Talking, swallowing, eating and controlling bladder and bowel function may become difficult.

A brain imaging test may also be ordered for diagnosis. For those at risk of developing HD, a blood test is also available to see whether the HD gene has been inherited. Not all people at risk choose to have this test.

Although there is no way to stop the progression of HD, medication can help control the movement problems and emotional difficulties of HD patients. These medications can come with unpleasant side effects, including fatigue, restlessness, and over-excitability. Daily exercise is important for HD patients, because those who exercise tend to feel better than those who do not. Good nutrition and adequate fluid intake are also important to prevent weight loss and dehydration.