Interesting! I have suspected for a very long time that the same three factors that affect longevity—diet, exercise, and happiness—are the dominant factors that predict Alzheimer's. If we we don't have good diets, exercise and happiness, then we're less resistant to bacteria and viruses that damage most organs.
I wouldn't be surprised if at the end of the day, they found that this study is right, that herpes does causally influence the creation of the plaques behind Alzheimer's, but that poor health hurts our resistance to herpes.
It has long been known that high cholesterol levels is correlated with the risk of Alzheimer's.