So I don’t know how well known or believed this is but I believe it. The cleaner we get as a species the more allergies we have. There is likely some good to having eaten dirt as a child. Little Z doesn’t seem to have any allergies and she gets plenty of contact with the dirt stuffs out here.
Anyhow, here’s a book review at ars techinica about An Epidemic of Absence: A New Way of Understanding Allergies and Autoimmune Diseases.