I recently ran across this article from ABC News in Australia reporting on a British research study which claims that cold sores triggered by the herpes simplex virus may be a "major" cause of Alzheimer's Disease.
The virus apparently creates a large amount of the same protein that comprises the plaque seen in the brain of an Alzheimer's patient.
The research team leader claims that this cold sore virus could be the cause of at least half of all Alzheimer's cases.
I suppose that the silver lining in this story is that, theoretically, a vaccine that prevents this virus could dramatically reduce the incidence of Alzheimer's cases. But the article indicates that there is currently no vaccine and the development of a effective vaccine is still "quite a long way ahead".