Some of the theories seem to hinge on HSV1 and ApoE4 (https://scholar.google.com/scholar?q=hsv1+apoe4) both being necessary to substantially drive up the risk. One example: https://www.sciencedirect.com/science/article/pii/S019745800...

I don't currently have access, but it'd be nice if someone who did could crack into the statistics of some of these.