It could be that autoimmune activity/genetics, even if it doesn't result in a specific disease, contributes to certain aspects of dementia. Take Celiac disease

It could be that autoimmune activity/genetics, even if it doesn't result in a specific disease, contributes to certain aspects of dementia. Take Celiac disease as an example. It only affects ~1% of the population, but ~30% of the population has the two genes (HLA-DQ2 and DQ8) that are most strongly associated with it and those genes are also associated with other autoimmune diseases. There are similar thoughts with respect to the hygiene hypothesis and autoimmune/immune-related disease. https://en.wikipedia.org/wiki/Hygiene_hypothesis#Overview https://en.wikipedia.org/wiki/Hygiene_hypothesis#Overview Other diseases, like Parkinson's, are also associated with autoimmune diseases. I suspect that there's some trade-off with respect to the effectiveness of the immune system/how well it deals with infections/cancer and the incidence of certain diseases/pathologies, but it's not something that's as straightforward as we would like it to be.