The existing research shows, in no particular order, all of the following:
1- A fetus exposed in the womb to bacterial toxins develops a hypersensitivity and an over-reaction to future exposure to that toxin. Since that toxin is everywhere (it makes up over half of the volume of common house dust for example) that exposure is inevitable.
2- Further exposure to the toxin results in reactions within both the central and peripheral nervous systems. The peripheral reaction (outside the BBB) has been observed to last a matter of hours. Within the BBB, however, the reaction has been observed going strong ten *months* later.
3- This reaction churns out cytokines - protective chemicals in an acute time frame but destructive in the chronic.
4- Cytokines are neuroactive and function as neurotransmitters. They change the way our brains work and how we think and how our bodies work.
5- Cytokines are perceived by the stress systems as a stressor and the endocrine system responds by releasing cortisol and other steroidal hormones - protective in acute situations and destructive in the chronic. See #3 above. They are also neuroactive and influence mental function. See #4.
6- Just as cytokines increase hormonal production, the increased hormonal levels increase cytokines in a feedback loop. Acute advantage but chronically destructive.
7- This all begins with the immune response and gradually draws in the endocrine. That beginning can be in the womb as described yielding young onset. It can begin with a virus as a young adult or it can begin with a natural part of aging yielding senior onset.
8- Once this reaction is established, a large number of stimuli can set it off. Mercury. Manganese. Agricultural dust. Gases released in the shower by ground water. Milk contaminated with the heat resistant toxins. Stress.
9- This ongoing inflammatory response can lead to increased permeability in the lining of the gut as well as in the much discussed BBB.
10- Secondary infections such as UTIs and bad teeth can kick it all into overdrive and rapid decline results.
11- Chronic stress serves as a constant goad to keep the process rolling.
12- Cytokines and stress hormones chronically encountered result in damaged neurons.

Simple.