Answer:
Scientist do not yet know how exactly Alzheimer's disease is caused. It is believed however that people with down syndrome will develope brain changes of Alzheimer's by age 40. The main risk of Alzheimer's disease is age. 10% of those over 65 and 50% of those over 85 have a chance of getting Alzheimer's.
Scientist believe that Amyloid cascade hypothesis is the main cause of Alzheimer's. In a study conducted with patients that had Alzheimer's half of them mutation that leads to over producing in the brain of a specific form called Abeta.