Okay, so I'm not really wondering that but it seems like studies continue to come out that indicate that it's better for children to be exposed to dirt and other irritants early in life so their immune systems will be stronger. A couple of new studies with rats not only grossed me out but also made me think that there must be something to this belief.  

Gritty rats and mice living in sewers and farms seem to have healthier immune systems than their squeaky clean cousins that frolic in cushy antiseptic labs.

The studies give more weight to a 17-year-old theory that the sanitized Western world may be partly to blame for soaring rates of human allergy and asthma cases and some autoimmune diseases, such as Type I diabetes and rheumatoid arthritis. The theory, called the hygiene hypothesis, figures that people's immune systems aren't being challenged by disease and dirt early in life, so the body's natural defenses overreact to small irritants such as pollen.

Dr. William Parker at Duke is so serious about this that he plans to build a 50 foot sewer in a lab to better control the experiment. Sounds fun for the neighboring labs.