Dementia is a brain disease which often starts with memory problems, but goes on to affect many other parts of the brain.
Dr Gill Jenkins last medically reviewed this article in November 2009.
It causes changes in mood, judgement, personality, and makes it difficult to communicate or cope with day to day tasks. It usually gets worse over time, which means that you have to rely on other people more and more.
Dementia can rarely start as early as 40 and affects about one in every 20 people over-65 have dementia. By the age of 80 about one in five are affected.