Introduction Dementia represents one of the greatest challenges of modern medicine related to the ageing of the global population. The growing number of elderly people is contributing to an increase in the number of patients with dementing diseases, creating significant public health implications. The pathogenesis of diseases such as Alzheimer's disease (AD), frontotemporal dementia (FTD), dementia with Lewy bodies (DLB) or vascular dementia (VaD) is not fully understood. Recent theories suggest a possible involvement of microorganisms in the development of dementia. Aim The aim of this review is to assess the current knowledge of the likely contribution of microorganisms to the manifestation of dementia and their potential role as diagnostic and prognostic factors in the future. Material and Methods Our review involved freely accessible databases: PubMed, Google Scholar, ScienceDirect, using keywords such as: dementia, microbes, virulent factors, neuroinflammation, neurodegenerative disorders. Results This review highlights the likely relationship between microorganisms such as Porphyromonas gingivalis and Helicobacter pylori and the development of dementing diseases such as Alzheimer's disease, frontotemporal dementia, dementia with Lewy bodies and vascular dementia. Conclusions Microorganisms including Porphyromonas gingivalis and Helicobacter pylori and their virulent factors are most likely to be involved in the pathogenesis of dementia diseases. Among these are lipopolysaccharide (LPS), gingipains and cytotoxin-associated protein A (CagA).