Bacterial and viral infections commonly found in periodontal disease may impact the brain either directly or though systemic signals to the brain. This may contribute to the development of Alzheimer's disease. Further evidence is needed to determine whether pathogens contribute uniquely to Alzheimer's disease.
Also, periodontal infection may elevate the systemic inflammatory response and thus contribute to existing brain and vascular pathologies that would impact brain function. Periodontal disease and Alzheimer's disease may also share genetic traits related to production of inflammatory mediators.
Watts, A.; Crimmin, E.M.; Gatz.M.: Inflammation as a potential mediator for the association between periodontal disease and Alzheimer's disease. Neuropsychiatric Disease and Treatment. 4(5):865-876, 2008.