Dementia A pathological condition that leads to a comprehensive deterioration of thinking. The person may lose the ability to construct speech, understand it, count, or simply navigate. Why does dementia develop? The best known and most common cause is Alzheimer's disease. But other illnesses and injuries also lead to the development of dementia. Suffer from this degenerative process in the brain, most often the elderly. But young people also suffer from dementia, accounting for 9% of cases.