Feasibility and Effects of Valaciclovir Treatment in Persons With Early Alzheimer's Disease

Inclusion Criteria: * Man or women, age ≥ 65 years * Ability to take a stand and to make and to sign an informed consent to participate in the study. This implies that a person with MMSE (Mini Mental State Examination) \< 18 will probably not be included. * Diagnosed with Alzheimer's disease or mild cognitive impairment due to Alzheimer's disease. At least one brain imaging examination should have been done (CT, MR, SPECT or PET/CT) and at least one objective finding should support the diagnosis beyond specific medical history. Reduced perfusion or reduced metabolism bilaterally temporally, hippocampal atrophy or pathological markers for Alzheimer's disease in cerebrospinal fluid is such findings. Persons with vascular brain disorders e.g. severe white matter changes or previous brain infarction will not be included but those with white matter changes considered normal for their age can be included. * Positive for anti-HSV (Herpes Simplex Virus) Immunoglobulin G (IgG) in plasma, i.e. carrier of HSV. * Hetero or Homozygote for allele 4 of gene Apolipoprotein E. * Stable over all medication including medication for Alzheimer's disease (rivastigmine, galantamin, donepezil or memantin) for at least one month. * No known allergy or oversensitivity against valaciclovir or aciclovir. * Ability to independently or by support from relative or other caretaker comply to study drug. Exclusion Criteria: * Renal insufficiency with estimated GFR (Glomerular Filtration Rate) ≤ 30 ml/min/1.73m2 * Ongoing treatment with anticoagulants (Warfarin, low molecular heparin or other anticoagulant agents). Antiplatelet agents in recommended dose are accepted (i.e. Acetylsalicylic acid 75 mgx1) * Life expectancy \< 1 year due to other comorbidity * Ongoing severe somatic condition that might interfere with the patients participation in the study (i.e. ongoing cancer treatment) * Ongoing illness that makes exams in a supine position impossible (i.e. severe heart failure, severe back pain). * Dementia diagnosis other than Alzheimer's disease, including Vascular dementia. * Other known neurological/neurodegenerative disorder (i.e. brain tumor, MS (Multiple sclerosis), ALS (amyotrophic lateral sclerosis)) * Claustrophobia or other contraindication for doing a PET/CT scanning. * Depression or other psychiatric illness that requires treatment (i.e. severe psychosis or other illness with equal grade of seriousness) * Dementia or cognitive dysfunction to such extent that an informed consent is impossible to obtain, corresponding to about MMSE-SR (Mini Mental State Examination-Swedish revision) \<18. * History of substance abuse (i.e. central nervous system stimulants or alcohol). Nicotine use is accepted. * Not willing to participate in the study.