There was an article on NPR Morning Edition, this morning about the Hygiene Hypothesis. A study in Sweden found that children in homes without dishwashers had 50% fewer cases of allergies including hay fever.

The assertion being that exposure to bacteria and other microbiota early in life mediate sensitivity later in life.

http://www.npr.org/blogs/health/2015/02/23/387553285/kids-al...

http://en.wikipedia.org/wiki/Hygiene_hypothesis