I don't think Alzheimers is new. As another poster said, until the last couple of decades nobody lived as long as we do now. My grandfather was once a magnificent figure of a man. In his 70s Alzheimers struck him down. By the time he died at 83 he had shriveled away to skin and bones and couldn't walk, barley talked and only remembered one person, my aunt. His father died at 45 and his grandfather died younger than that. Had they lived long enough who knows? It's been said that prostate cancer is more common now because men simply live long enough to get it now.
A few days ago I offended a co-worker when I told her flatly that I'm sick of hearing about AIDS and am far more worried about unfashionable diseases like cancer, diabetes and Alzheimers. She was aghast and hasn't spoken to me since.