Research published today in the journal “Neuron”, from analysis of DNA and RNA in Alzheimer brains, shows that Herpes virus HHV-6A and 7 (as well as the cold sore Herpes HSV-1) are present at higher levels in Alzheimer brains than normal brains, and that the level is associated with severity.
It is posited that the virus suppresses a microRNA which usually turns off a specific gene. Indeed when they bred mice deficient in that mRNA, they found the mice developed abundant amyloid plaques – the hallmark of Alzheimers.
Alternatively it may be that amyloid plaque is generated as a defence by the brain against the virus, but that the immune response cascades out of control.
A lot more research is needed to clarify, and unfortunately the Alzheimer research community is currently rather hostile to the idea that microbes are involved in the disease process. (Interestingly the researchers were not looking for viral implication, it just showed up in the data.)