This article supports, in a way, my comments about bacteria, disease, viruses, etc. being required to develop a good healthy immune system. It doesn't however explain why immigrants who have been raised in third-world countries would develop illnesses or syndromes when they move and change diets and environments. I have seen this in the Native communities nad among the Eskimo people. They will live heart disease free lives with cholesterol levels over 400 sometimes, but when they move to "civilization" and change diet and environment, they develop heart disease and diabetes. Strange..... This study seems to find that parasite infestation if required to avoid food allergies. https://www.bbc.com/news/health-46302780