Eating dirt is good for you.

I have always attributed my robust immune system (I don't get colds) to the fact that when I was 1-5 years old I was crawling or running around barefoot in the hot Texas summer, eating dirt, pill bugs, and dog shit. Brody does a nice summary of what is now the accepted view: that exposure while young to a diverse array of bacteria, viruses, and worms trains the immune system in what is and is not important, essentially programs it for later adult life. Children raised in super hygiene environments are more likely to develop allergies, asthma, and autoimmune disorders.