Erika, I'm glad you brought up the situation where people do not have much MS in tropical climes, but you also said they didn't have much MS in far north climes. Well, I know you didn't set forth the latitudes here, but I think some northerly people do get lots of MS, particularly in Sweden and Finnland (I have no info about other Scandinavian countries or Alaska). I am wondering if this variation is a genetic thing. I am wondering if some ethnicities get it more, and they are from certain climes. I read once from a good researcher that there is a very high level in Sweden, but also a high level in Oregon's Willamette Valley, and he attributed this to arsenic in the soil, put there naturally by vulcanism. But also some ethnicities may have a harder time combating the poisons such as this.
My "other" disease, at least the first "other", the one which also has neuro symptoms, that is Porphyria, DOES have a very hard time combatting poisons such as this arsenic, and Porphyria is high in Sweden and in the Willamette Valley in the same places where MS is high (this is from old research which I don't have, so I am writing from memory, material we got on a forum from a European researcher called Hans, who was sharp as a tack but left because we fought too much). My paternal aunts, who had MS diagnoses had a mother from a far north clime, but they also lived in the Willamette Valley. I think they had Porphyria too, but of course I'm speculating, but I am doggone sure about one of them, Wilhelmina. Her sister Ethelwynne died of kidney failure at 30 and I didn't know her, so that's another story.

Anyway, as Swank said, some people may have something missing in their blood which makes it harder for them to detox poisons. Annesse has suggested pancreatic enzyme deficiency, but has not shown this to be true to our satisfaction. There could be another factor; it could be genetic, as Porphyria is. Or this could be the cause in some cases of MS, those caused by chemicals rather than an auto-immune disease.

I am rambling but wanted to get some of these thoughts out, because I think they are worth considering even if they are specs.