The triple-hit hypothesis of Alzheimer's disease: blood-brain barrier breakdown, infection, and neuroimmune activation as a unified etiological framework.

Studies of Alzheimer's disease (AD) have long been dominated by the amyloid cascade hypothesis, although mounting evidence suggests that amyloid-β (Aβ) deposition might be a late downstream event, rather than the initiating trigger of AD. Here, I propose a unifying Triple-Hit Hypothesis in which AD develops through a sequential interaction among three causative processes that have been individually implicated before in the onset of Alzheimer's disease: (1) early blood-brain barrier (BBB) breakdown, (2) entry or reactivation of microbial agents within the brain, and (3) maladaptive innate immune responses that produce chronic neuroinflammation and Aβ accumulation. Converging data from human imaging, neuropathology, infectious disease studies, genetics, and vascular medicine could suggest that these three processes, which were in some previous studies linked to AD, might not be independent but rather that their temporal synergy could drive disease progression. This new conceptual framework integrates long-standing but fragmented lines of evidence and, if confirmed by experimental studies, offers the possibility for the development of new diagnostic biomarkers, new therapeutic entry points, and prevention strategies for AD. I argue that understanding AD as a disorder of the blood-brain barrier, immunity, and host-pathogen interactions should be taken into account in future research on the etiology and clinical progression of AD.