This is astounding new research. (originally posted Oct 11, 2008)
http://www.sciencedaily.com/releases/2008/09/080921162048.htm
http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature07336.html

We already knew that farm kids who grow up with 4 siblings, 6 animals in the house, 47 animals in the barn and hay in their hair have less allergies than kids who grow up in pristine urban apartments, who breathe filtered air, don't play in the dirt, and are regularly bathed in anti-microbial soap. Now there's reason to believe that getting dirty might even protect you from diabetes....

Mercola says that in addition to all of this, your kids are less likely to develop type I diabetes if they get plenty of sunshine. Apparently vitamin D deficiency is related to the development of auto-immune disease.
http://articles.mercola.com/sites/articles/archive/2008/10/11/friendly-bacteria-protect-against-type-1-diabetes.aspx?source=nl