Huntington's Disease (HD) is a hereditary illness that causes damage in a certain part of the brain. This stops the brain and body from working as well as they used to, and affects a person's movements, thoughts and behaviour. People generally start to develop symptoms of HD between the ages of 30 and 50.

The part of the brain that is affected is called the basal ganglia (say: bay-sul gang-lee-ah). This is a very important part of the brain that coordinates a lot of the information that's whizzing around, and makes sure it gets to the right part of the brain. HD causes brain cells in the basal ganglia to deteriorate and die. This means that it can no longer co-ordinate information as efficiently, so the messages being passed through the brain don't always get to the right place, or get lost along the way.

Think of your mobile phone. Sometimes you get a signal, no problem. But at other times, the signal gets cut off halfway through, just when you were getting to the good part of the gossip! At other times, you cannot get a signal at all.

Having HD is a bit like this, sometimes the messages get through and other times they only get a little of the way through. Other times they just don't get through at all. So some days are better than others. Some things affect the messages getting through. For example, if the person is feeling stressed this makes it harder for the messages to get through and things may become worse.

The brain cells in the basal ganglia deteriorate over a long period of time, 15-20 years, and during this time the affected person slowly loses different skills and abilities. This happens gradually and they can be reasonably well for a long time.

For more info on how the brain works, check out www.kidshealth.org