Due to difficulties with access (nobody wants to access the brain without good reason) and the expected effect of the blood-brain barrier, infection of the brain or a harmful microbiome dragging down health is likely under-diagnosed and poorly understood.

Analysis of deceased patients indicated that there is a difference between sets of microbial species that can be grown from brain samples taken from people who died with dementia, and people who died without dementia. Some cases of dementia have proven to be medically reversible with anti-microbial treatments.

It seems that understanding how microbes reach the brain and how they influence it (and how to influence them) is a promising field of study.

P.S.

From other sources, not the article, I recall that dental health - especially in old age, when the immune system becomes less capable - has a correlation with brain health. (It would not be a big leap to guess: there might also be a correlation between brain health and processes in the nose, eyes and ears.)