For me, the statement "stress is the one thing that clearly makes a difference" is probably the most important clue we have. It leads to several interesting points:
1) Stress reactions lie within the domain of endocrinology, not neurology. That alone means trouble for us. We fall between the cracks.
2) Stress research reveals a picture of a balancing act built around a lifelong pursuit of homeostasis ("health" or "balance"). It is like balancing on one foot, even for normal folk.
3) Certain things can make it harder to balance on that foot. The prenatal bacterial toxin hypothesis incorporates one.
4) As time goes on it gets harder and harder as the system fatigues. As we age we encounter secondary insults such as pesticides. Where a healthy system might be able to maintain balance, we start to wobble.

Since each of us is at a different point in that continuum and since we encounter different secondary insults, each of us is different in presentation.

Except for the response to stress. There is something unusual about our relationship to it. As we discovered earlier, a heck of a lot of us had high stress childhoods, jobs, etc. and held up well (or seemed to). Somewhere along the line we crossed a threshold.

One interesting thing is that this view makes room for other theories as "secondaries". The BBB idea fits in well, for example. Stress increases inflammation which increases BBB permeability and so on.

So, if we want to get into that let's jump to its own thread. In fact, I will paste this over for a starter.