How does Alzheimer disease differ from dementia?

Alzheimer disease is a brain disease. It is the most common cause of dementia. Among people over the age of 65, Alzheimer disease accounts for over two-thirds of dementia cases. Dementia describes a clinical syndrome of progressive loss of thinking, memory, and other cognitive abilities impairing daily function. The most common symptom of dementia caused by Alzheimer’s disease is a gradual worsening of memory.