Purpose and aim: To map differences in the blood microbiome in different types of prevalent dementia compared to cognitively healthy controls, and to determine the impact of variations in the blood microbiome on the risk of incident dementia in relation to known risk genes, comorbidity and life-style factors. Method: Amount and genera of bacteria in blood samples will be determined by 16S-sequencing of bacterial DNA, and studied in relation to incident and prevalent dementia, co-morbidity and known risk genes for dementia in two high-quality cohort studies separately for verification. The results will guide a new data collection of peripheral blood mononuclear cells from dementia patients and cognitively healthy controls, in which 16S-sequencing of bacterial DNA will be performend in sorted cells. Alzheimer's disease, Parkinson's disease and Vascular dementia will be studied. Relevance: The results may reveal novel information on routes of infection between known peripheral low-grade infectious loci and the brain in dementia. This information will contribute with improved understanding of different contributing factors and mechanisms in dementia, which likely has a diverse and multi-factorial etiology, and may drive a new research field on anti-microbial and immune-modulating therapies in dementia.