He said that public health measures like cleaning up contaminated water and food have saved the lives of countless children, but they "also eliminated exposure to many organisms that are probably good for us." "Children raised in an ultraclean environment," he added, "are not being exposed to organisms that help them develop appropriate immune regulatory circuits."
One of the decisions we made even before Ollie was born was that he was going to be a dirty kid. We wash our hands often with non-antibacterial soap and water, especially after being on the subway, but otherwise don't worry about it much. I can count on one hand how many times I've used the antibacterial hand sanitizer that seemingly comes bundled with toddlers these days.
Update: See also The Germ-Phobic Mommies.