In dirt.
Instapundit asked:
Is this an argument in favor of daycare?
In regard to a study published in Nature showing early exposure to germs in childhood confers more immunity. This offers more proof for a known.
It isn’t, however, a justification of childcare. The caption of a picture of kids playing in the mud reads:
Dig in: eating dirt and playing in the mud are thought to confer protection from allergies and asthma.
Precisely. Studies have always shown farm kids grow up healthier. Kids now either don’t have as much access to dirt or their parents don’t allow them to get … dirty. Rolling around in the mud has always been an excellent means of play (that’s why we have washing machines, folks), and now you can argue with your neighbors that it makes kids healthier, too! Win-win for being the cool mom on the block!
More from Nature:
In a study published online today in Science1, the researchers show that in mice, exposure to microbes in early life can reduce the body’s inventory of invariant natural killer T (iNKT) cells, which help to fight infection but can also turn on the body, causing a range of disorders such as asthma or inflammatory bowel disease.
The study supports the ‘hygiene hypothesis’, which contends that such auto-immune diseases are more common in the developed world where the prevalence of antibiotics and antibacterials reduce children’s exposure to microbes.
“We as a species are not exposed to the same germs that we were exposed to in the past,” says study co-author Dennis Kasper, a microbiologist at Harvard Medical School in Boston, Massachusetts.
The researchers induced two groups of mice — germ-free (GF) mice, which are raised in a sterile environment, and specific-pathogen-free mice raised under normal laboratory conditions — to develop forms of asthma or ulcerative colitis. GF mice had more iNKT cells in their lungs and developed more severe disease symptoms, indicating that exposure to microbes was somehow influencing iNKT cell levels and making the GF mice more susceptible to inflammatory diseases.
Go out. Play with your kids. Let them get dirty. It’s good for them developmentally but it also makes them stronger!
UPDATE: linked by Pundette as a Recommended Read. Thanks!
And Chris Wysocki at Theo Spark. Thanks!
Politicaljunkie Mom 
Sounds more like an argument for free ranging. Argument for day care is that mommy has to work.
My dad has been a plumber for thirty years. He always found that houses that did not filter their water were more healthy. I found the same.
We moved from Seattle to Forks (yes the Twilight Forks) and my kids asthma and allergies cleared up. Now Forks has better air by an order of magnitude but the difference in the kids was greater than that.