[Note: this was published in the "Letters" columns of SN.]
The article titled "Do More Infections Mean Less Asthma?" (SN, 8/26/00, p.134) suggests that exposure to microbes in early life helps develop a child's immune system.
Speaking of dietary habits among 18th-century "Scotch-Irish" immigrants to the back-country regions of colonial America, David Hackett Fischer notes in Albion's Seed (1989, Oxford University Press): "The quality of butter was believed to be improved in proportion to the number of human hairs embedded in it. 'The mair dirt the less hurt,' Appalachian housewives liked to say."
Perhaps they were on to something