Letenneur et al. [30] surveyed a group of people aged 65 over a 14-year period and found, using serum anti-HSV1 IgM as a marker of recent HSV1 reactivation in the peripheral nervous system, that those who experienced HSV1 reactivation had an increased risk of developing AD compared with those who were IgM-negative. This evidence concerns the gene CD40LG and Alzheimer disease.