It's quite possible that it's environmental suppression of immunity, aging and/or degradation of the blood brain barrier that permits the viruses to make their way topside. There's some evidence that the amyloid plaques actually exhibit antimicrobial activity and may be an immune response to infection. [1]

In my unsubstantiated opinion it's likely to be herpesviridae that make their way to the brain due to their affinity for nerve tissue and the fact up to 80% of Americans have HSV1. Separately 50% of Americans have cytomegalovirus, another herpes virus. And yet more have HSV6 and HSV8. It's safe to assume over 90% of Americans have some latent herpes-family virus.

It also explains why acyclovir and valacyclovir dramatically reduces the risk of developing dementia.

[1] https://www.nature.com/articles/s41598-024-55423-9

[2] https://www.infectiousdiseaseadvisor.com/news/antivirals-for...