I am very, VERY convinced by the hygiene hypothesis, but there's just one small question mark which you describe exactly: why isn't it too late after the age of about 15?
What we are seeing involves two completely different situations.
1. Evidence leading to the postulation of the hygiene hypothesis pointed towards "evolutionary normal" conditions before puberty "teaches' or "matures" the immune system, so that in adulthood your immune system behaves appropriately, even without the presences of parasites. I think a "proof" is to be found in the difference in incidence between developed and non developed populations.
2. The other situation we are familiar with involves adults who hadn't experienced "evolutionary normal" conditions pre puberty and had developed immune dysfunction in adulthood and were then exposed to parasites.
#2 is among the reasons that I point out that the alleviation or opposite of the cause is not always the cure. Considering that one of the key factors of the hygiene hypothesis is enabling a "mature" immune system for entrance into adulthood, it's not expected, or fair to expect to be able to "teach" the immune system after adulthood. When I said that we were lucky that helminth treatment worked on adults already experiencing immune dysfunction, I meant that we were lucky to have found positive results and not that the adult immune system could be trained at that point, ie: one time treatment.
I have heard of some adults with chronic asthma experiencing what amounts to a "cure" after exposure to parasites, (remember the guy who walked barefoot through latrines?), but the fix isn't permanent and re–exposure is often necessary
One thing that has become eminently clear is the inadequacy of the terms "cure" and "disease". They are so vague and all encompassing as to be absolutely worthless in what we would like to use them for. I use the word "cure" about 15 different ways, you and everyone else does and none of us have any way of knowing what the other means when they use it. Considering your above use of the word "cure", I again argue that he was an adult, already experiencing immune dysfunction. He did experience positive results but because of reasons mentioned above, at this point it's unfair to expect the opposite of the cause to be the cure in this case (if "cure" is considered to be a one time treatment.
Further muddying the situation, for someone trying to understand, is the fact that the T trichiura (human whipworm) lives up to 20 years in humans......providing the "cure" for 20 years. For someone 70 years old that would assuredly be a "lifetime" treatment, but through the rest of their lifetime they would have parasites controlling their immune system, as opposed to someone who had experienced evolutionary normal conditions and wouldn't need parasites in their system to avoid immune dysfunction.
so I just wonder if there's something else during childhood which establishes the immune system change permanently for life, (hormones, diet etc. – all the usual suspects). After all, not every child who lives free of parasites will go on to develop disease, so I'm guessing that there must be some other contributing factor.
My above arguments have pretty much answered this to the best of my ability. With that said, I think it's a situation involving what I can't help but think of as a "predominant" predisposition. One key factor without which it would be impossible for the disease process to start, and after that other factors come into play. It's important to always realize that the only view we've ever had of MS is AFTER it's started, so we're seeing evidence of all the factors and makes it seem like stopping the disease involves some nightmare situation of having to know and correct multiple different factors, when in reality we only need to reverse the key factor.
Have I mentioned that those who cannot learn from history are doomed to repeat it?
Just keep in mind the long frustrations involved in the search for the causes of the vitamin deficiency diseases and that those people were convinced that the cure HAD to be terribly complicated. Now think about how many lives could have been saved by eating an orange.
Judging by the odds, which in the absence of fact otherwise, is the best way to go, MS is not as complicated as it has always "seemed". Despite initiation of and adherence to the peer review process, which was found necessary to get MS research away from what "seemed" to be logical, MS research is still primarily driven by what "seems" and what has "seemed".