Recently studies are starting to show that the increase in asthma and allergies may be related to the exposure (or lack thereof) of children to mild irritants when they are young.
It seems that hypercleanliness is not a good way to raise most organisms.
Nobody has gone so far as to recommend feeding kids from dumpsters or anything, but in general, kids playing in the dirt, and wiping their noses on their sleeves may be more healthy habits for long term success than it would appear. And the "two second rule" for anything that falls on the floor may have to be extended