Alzheimer's disease: the infectious hypothesis

Several hypotheses are proposed for understanding the Alzheimer's disease (AD) pathological mechanisms, mainly the amyloid theory, but the process inducing A beta peptide deposit, tau protein degeneration, and ultimately neuronal loss, is still to be elucidated. Alteration of the blood-brain barrier and activation of neuroinflammation seem to play an important role in AD neurodegeneration, especially in the decrease of A beta peptide clearance, therefore suggesting a role of infectious agents. Epidemiological and experimental studies on cellular or murine models related to herpes simplex virus (HSV), spirochetes, Chlamydia pneumoniae or Borrelia, and systemic inflammation are reviewed. A beta peptide or tau protein could also behave like a prion protein. Infectious agents could thus have an impact on AD by direct interaction with neurotropism or systemic inflammation. Although the results of these studies are not conclusive, they may contribute to the understanding of AD pathology