Allergies and the Hygiene Hypothesis
The "hygiene hypothesis" is the theory that the comparatively high rates of asthma and autoimmune diseases in developed nations are due to the fact that our immune systems weren't challenged enough when we were young. If a child lives in a home that is scoured with anti-bacterial cleaners and never comes into contact with microbes, animals or other "dirt," the immune system will atrophy like a muscle that isn't used, or so the thinking goes. It is also thought that certain parasites and childhood infections may trigger mechanisms that help prevent asthma. Thus overly clean environments may be detrimental rather than beneficial to the health of young children.
While this does explain overall higher rates of asthma in the United States, it does not fully explain all aspects of increased asthma rates, such as why poor inner city children have higher asthma rates than their well-off suburban counterparts. Not everybody is convinced that the hygiene hypothesis tells the truth, at least not the whole truth, about the development of our immune systems. One study counters some of the tenets of the hygiene hypothesis by showing that in some cases exposure to animals and infections can actually increase the risk of asthma in children.
