As people age, the blood-brain barrier deteriorates and becomes leaky, allowing infections to get into the brain. The brain responds by trapping the offending microbes in protein cages (made up of beta amyloid proteins). The leftover plaque is a hallmark of Alzherimer’s
The plaque is not cleared out efficiently by the brain and causes a pathway (involving tangled tau proteins) that cause the brain to become inflamed and killing many nerve cells.
A small subset of the population have a gene (ApoE2) that are good for sweeping out plaque. These people have a low risk for late-onset Alzheimers.