Excercise your immune system (The danger of being too clean)When I was in high school, my friend spent months saving up enough money to buy a mini-stereo system, which was pretty much the most 'totally boss' thing you could have at the time. I helped him set the system up, and he popped in his favorite cassette tape. As we waited for the tape to start, he adjusted up the volume. And then the system blasted out a gut-shaking noise approaching the jet-engine level on the decibel scale, probably permanently damaging our hearing before he shut it down.
The question is: why did my friend turn the volume up to such an unreasonable level? He was used to a certain level of background noise from his previous boom box, so as we waited for the tape to start, he kept turning up the volume, waiting for the same background noise. Of course the new system had much better electronics, so he ended up turning the gain to an unreasonable level.
There's some evidence that our immune systems work the same way--when you're first born, your body expects a certain level of background allergens to be present, and if the body doesn't find them, it keeps turning up the gain, becoming more and more sensitive. Unfortunately, this means that when someone is finally exposed to something with allergen potential, like peanuts, the body overreacts with tragic results.