No-one is one hundred percent sure about what causes HD. We are fairly certain that the damage in the brain is caused by a build up of a protein (a type of body chemical) called Huntingtin. The cells in a certain part of the brain slowly become sick due to a build up of this protein over a long period of time - it is when the build up of this protein becomes too much that the cells can no longer work as well as they used to and become sick and start to die. This damage to the brain cells is what causes all the changes in HD.

This is why most people only start becoming unwell when they're older - it takes that long for the protein to build up in the brain cells and make them sick. This means that people are well for a long time before any signs of the disease start to show. So they live normal lives like everyone else - they have careers, they get married, travel - do all the usual stuff basically!