Dr. Larry Dossey is an advocate for what has become known as the 'Hygiene Hypothesis.' The theory argues that excessive cleanliness and a lack of exposure to various microorganisms can affect the developing immune system of children and make them more susceptible to autoimmune diseases such as asthma. For more, see HealthLink(1) and Wikipedia(2).

1. http://healthlink.mcw.edu/article/1031002421.html
2. http://en.wikipedia.org/wiki/Hygiene_hypothesis