HHV-6 is much more prevalent in AD than in age-matched normal brains (70 vs. 40%) and overlaps extensively with HSV-1 in AD brains (Bourgade et al., 2015), but HHV-6, unlike HSV-1, is not directly associated with APOE-ε4 in AD (Itzhaki, 2016), and its pathogenic mechanism has not yet been elucidated. Here, APOE is linked to Alzheimer disease.