A degenerative brain disease characterized primarily by progressive dementia. Type 2 has a late onset – starts after the age of 65. It is believed to be caused by a combination of genetic mutations and environmental and lifestyle factors. The condition occurs when there is excessive production of a toxic protein (amyloid beta peptide) in the brain which collects into clumps (amyloid plaques) in the brain. These plaques cause damage to nerve cells in the brain. See also Alzheimer disease type 2