The Defence of Dirt

"As the Duchess of Windsor once remarked, one can never be too thin or too rich—wisely, she did not add too clean. The hygiene hypothesis aims to explain why some people have allergies and some do not, and why the prevalence of allergic disorders has been increasing over the past century. It suggests that the modern obsession with cleanliness may be counterproductive; in childhood, at least, it may encourage the development of allergic disorders..."