There is exciting research looking at the possible connection between infections and imbalances in brain function.  It appears that low-level infections can potentially cause or contribute to brain disorders such as Alzheimer's, OCD, Anxiety, Tourette's, Migraines, and more.  More research needs to be done, but the links are absolutely fascinating.  In the case of Alzheimer's, the idea is that a microbe such as herpes virus (HSV1 is specifically the one that causes cold sores) may directly impact nerve cells in the brain.  Another mechanism postulated is that bacteria such as strep may cause antibodies to be produced that cross-react with the brain.  In the case of strep, there are antibody profiles that can be ordered by physicians which can help confirm or rule out the extent to which the immune system is reacting to the bacteria.  One obvious thought that comes to mind is why do these microbes affect some people and not others?  Differences in genetics are thought to playa role, and to me the obvious difference would be each person's individual ability to control infection, which of course involves overall health and immunity.