What percentage of people in their 60s and older have that virus? If 90% of the total population has it, then it being in 90% of the plaques causing Alzheimer's isn't very significant.
http://www.google.com/search?hl=en&source=hp&ie=ISO-8859-1&q=hsv-1+alzheimer%27s
I think your point is really important. When I did a very small search about HSV-1, on the first page of search results was a link to an article stating that a huge percentage of the population has HSV-1 DNA in their teardrops. Apparently, after infection with a virus, it can appear virtually anywhere in the body??