Letenneur et al. [55] 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 PNS, that those who experienced HSV1 reactivation had an increased risk of developing AD compared with those who were IgM-negative. Here, CD40LG is linked to Alzheimer disease.