In short, there are good reasons at this point to believe that amyloid-β's primary function is as antimicrobial peptide, and thus various infections may cause the seeding of amyloid-β deposits. These deposits may then persist for years beyond any useful benefit, especially if the brain's clearance mechanisms are impaired.
Note that, per [2]:
Importantly, in the antimicrobial protection model, neurodegeneration is not mediated by pathogen activities that directly kill neurons. Rather, Aβ innate immune pathways targeting pathogens mediate the AD [Alzheimer's Disease] neuropathogenesis that leads to widespread neurodegeneration. Thus, our model is consistent with the amyloid cascade hypothesis and overwhelming data showing the primacy of Aβ in AD pathology.
[1]. Dominy et al (2019). Porphyromonas gingivalis in Alzheimer’s disease brains: Evidence for disease causation and treatment with small-molecule inhibitors. https://doi.org/10.1126/sciadv.aau3333
[2]. Eimer et al (2018). Alzheimer’s Disease-Associated β-Amyloid Is Rapidly Seeded by Herpesviridae to Protect against Brain Infection. https://doi.org/10.1016/j.neuron.2018.06.030
While it may be considered andectoal, this fits with what i've seen in my family. In my mind and from what i've read it seems to be more of a combination of factors - diabetes, an APOE gene, toss in a little depression/loss of purpose/retirement/isolation, etc
I find the studies with gamma interesting: https://www.alzheimers.gov/clinical-trials/gamma-induction-a...
I think it would be good to bring more attention to noticing when things start to slip, because as things progress there can be less openess to trying new things/focus on maintaining state.