Starting from pivotal post-mortem studies showing the presence of HSV-1 DNA in those AD brains bearing the ε4 allele of apolipoprotein E, (a genetic risk factor for AD) [7,8], a recent multidisciplinary study [9], as well as population-based clinical ones [10,11,12,13,14,15] supported the hypothesis that periodic reactivations of the virus reaching the central nervous system (CNS) may predispose the brain to AD. This evidence concerns the gene APOE and Alzheimer disease.