We studied the effects of systemic infection on brain cytokine level and cerebral vascular function in Alzheimer's disease (AD) and vascular dementia (VaD), in superior temporal cortex (BA22) from AD (n = 75), VaD (n = 22) and age-matched controls (n = 46), stratified according to the presence or absence of terminal systemic infection. Brain cytokine levels were measured using Mesoscale Discovery Multiplex Assays and markers of cerebrovascular function were assessed by ELISA. Multiple brain cytokines were elevated in AD and VaD: Interleukin (IL)-15 and IL-17A were maximally elevated in end-stage Alzheimer's disease (Braak tangle stage V-VI) whereas IL-2, IL-5, IL12p40 and IL-16 were highest in intermediate Braak tangle stage III-IV disease. Several cytokines (IL-1β, IL-6, TNF-α, IL-8 and IL-15) were further raised in AD with systemic infection. Cerebral hypoperfusion, indicated by decreased myelin-associated glycoprotein: Proteolipid protein-1 (MAG: PLP1) and increased vascular endothelial growth factor-A (VEGF), and blood-brain barrier leakiness, indicated by raised levels of fibrinogen, were exacerbated in AD and VaD, and also in non-dementia controls, with systemic infection. Aβ42 level did not vary with infection or in association with brain cytokine levels. In controls, cortical perfusion declined with increasing interferon-γ (IFN-γ), IL-2, IL-4, IL-6, IL-10, IL-12p70, IL-13 and tumour necrosis factor-α (TNF-α) but these relationships were lost with progression of AD, and with infection (even in BS 0-II brains). Cortical platelet-derived growth factor receptor-β (PDGFRβ), a pericyte marker, was reduced, and endothelin-1 (EDN1) level was increased in AD; these were related to Aβ level and disease progression and only modestly affected by systemic infection. Our findings indicate that systemic infection alters brain cytokine levels and exacerbates cerebral hypoperfusion and BBB leakiness associated with AD and VaD, independently of the level of insoluble Aβ. Our findings highlight systemic infection as an important contributor to dementia, requiring early identification and treatment in the elderly population.