Autoimmune diseases, including type one diabetes, multiple sclerosis, rheumatoid arthritis, and Crohn's disease, are pretty much limited to the developed world. This is almost certainly due to environmental factors, as the populations aren't that different. One possible cause of autoimmune disorders is the lack of parasites. The claim is that for as long as people have existed there have been parasites that the immune system needed to fend off, but in the developed world parasites have been mostly eliminated. This would mean our immune systems had evolved to be more active than is now needed, so they end up working against healthy parts of the body instead of parasites. The 2006 paper Parasitic worms and inflammatory diseases gives a good summary of this argument and some supporting evidence. [1]

It seems to me we ought to be able to test this. There are currently charities that focus on deworming people. With a small investment, one ought to be able to track rates of autoimmune disease among villages where you do full deworming compared to villages where you'd changed nothing. [2]

[1] This idea is part of a more general hygiene hypothesis which claims that we need some amount of getting sick in order to stay healthy overall. It's used to explain why diseases like asthma, eczema, hay fever, and autism are becoming more common in the developed world.

[2] This is not to say that I believe money spent on deworming is wasted or even poorly used. Autoimmune diseases are rare even in places where almost no one has parasites. Parasites are probably, on balance, a big negative.