AFAIK a sticky plaque builds up in brain tissue, normally lymph carries this plaque away but when the lymph system deteriorates as in old age, it is overwhelmed and no longer effective.
There is a strong genetic link, but environmental toxins can also play their part.
Personality deteriorates alongside memory, as brain function becomes impaired. If you believe in reincarnation then the true Self will be reincarnated, if not they've gone forever.
Alzheimers is fatal, what starts as memory problems and erratic behaviours ends in loss of body function as the brain continues to deteriorate.