Hope this hasn't been posted before?
www.usatoday.com/story/news/2016/05/26/harvard-researchers-unveil-new-alzheimers-theory/85004894/"...but that some people’s brains may over-react to or get overwhelmed by a variety of pathogens, including chlamydia, herpes and the bacteria that causes Lyme disease, Tanzi said. It’s possible that younger people’s brains can handle such issues, but some older ones can’t."