Here is another study
Think again about keeping little ones so squeaky clean
"Contrary to assumptions related to earlier studies, our research suggests that ultra-clean, ultra-hygienic environments early in life may contribute to higher levels of inflammation as an adult, which in turn increases risks for a wide range of diseases," said Thomas McDade, lead author of the study, associate professor of anthropology in Northwestern's Weinberg College of Arts and Sciences and a faculty fellow at the Institute for Policy Research.
Relatively speaking, humans only recently have lived in such hyper-hygienic environments, he stressed.
I had all this explained to me recently by an immunologist and it is more complicated than it sounds in terms of balancing immune responses....but the overall picture is that we were not designed to live in such sterile conditions. I've heard of a doctor who is quite happy when his toddler licks the conveyor belt at the supermarket checkout.