This episode is presented by eBay.

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on things you love. You can't help but say when it's eBay. It excludes vehicles and business

sellers.

Welcome to Science Talk, the more or less weekly podcast of Scientific American,

posted on April 6th, 2011. I'm Steve Burski. This week on the podcast. The hypothesis is that as we make the shift from dirt to sterile, that you are changing the direction of your

immune response. This causes diseases. That's Kathleen Barnes. We'll hear from her and we'll test

your knowledge of some recent science in the news. Kathleen Barnes studies what's called the

hygiene hypothesis at the Johns Hopkins School of Medicine in

Baltimore. She presented some of her research at the recent meeting of the American Association

for the Advancement of Science in Washington, D.C., after which we sat down to chat.

First, tell me what is the hygiene hypothesis for people who haven't heard of it?

So the hygiene hypothesis, I guess, simply stated, is this notion that as human society has morphed from a developing environment,

what we would consider a developing world environment into the developed world.

There have been radical changes in our environment.

Changes associated with the size of family, so going from many to fewer siblings.

The idea being that with fewer children in the house, there's less opportunity for exposure to

viruses. The idea that we move from a rural to an urban environment, that we have moved from

that situation where we're exposed to microbes, one of the best examples in asthma being the idea that we're

exposed to endotoxin that is a byproduct of the livestock and farms to moving to an

...