There are scientific studies that show some merit in the idea that our immune systems need practice.
In this PBS series Pediatrician Erika Von Mutius says that exposure to some level of microbes may allow our immune system to learn how to tolerate allergens as well as fight infections, bacteria and viruses.
In a similar vein this Healthlink article from the Medical College of Wisconsin, asks the question Are We Too "Clean" for Our Own Good?
The Dirt on Germs | Orion magazine talks about the studies by Dr. Von Mutius and others and states,
"Children who had lots of siblings, who lived on farms, had cats, or went to daycare in their first year were discovered to do best at avoiding allergic diseases. Even children whose mothers had lived on farms during their pregnancies were less likely to become allergic. The children most likely to develop allergies and asthma were children who lived in cities, did not go to daycare, had no pets, washed their hands more than five times a day, and bathed more than once a day."
This article in the "American Journal of Respiratory and Critical Care Medicine has a different point of view, declaring that The Increase in Asthma Cannot Be Ascribed to Cleanliness.
The authors point out that overeating and lack of physical activity are not good for people. George Carlin already covered that idea too...you can find the video on YouTube.
