In clinically defined AD and biomarker positive MCI cases who are HSV seropositive (n = 130; NCT03282916), an 18-month valacyclovir (4 g/daily) treatment will be examined with cognition as the primary outcome, PET-defined amyloid and tau as secondary outcomes, and cortical thinning, olfactory deficits, and antiviral titers as exploratory outcomes. Here, MAPT is linked to Alzheimer disease.