Cerebral hypoperfusion—indicated by decreased MAG:PLP1 and increased vascular endothelial growth factor-A (VEGF)—and blood–brain barrier leakiness, indicated by raised levels of fibrinogen, were exacerbated in Alzheimer’s disease and vascular dementia patients, and also in non-dementia controls, with systemic infection. This evidence concerns the gene PLP1 and Alzheimer disease.