Many parents, quite reasonably, worry about germs and dirt finding their way into a child's mouth. But many have also heard in recent years of the hygiene hypothesis," which holds that some exposure to germs and microorganisms in early childhood is actually good for us because it helps develop the immune system. Jack Gilbert, the director of the Microbiome Center and a professor of surgery at the University of Chicago ... was one of the authors of a well-known 2016 study ... which compared the immune profiles of Amish children, growing up on small single-family farms, and Hutterite children, who are similar genetically but grow up on large, industrialized farms. The Amish, living in an environment ... full of barnyard dust, had strikingly low rates of asthma. Since understanding that microbes cause disease, human beings have tried as hard as possible to wall off their bodies from the microbial world of bacteria, viruses and fungi. A study published in 2016 ... profiled the microbial development of a group of babies in the United States, examining the ways in which their bacterial populations were affected by mode of birth, by formula feeding versus breast-feeding, and by antibiotic exposure. What we have learned, Dr. Gilbert said, is that early life exposure to microbes can shape not only the immune system, affecting a child's likelihood of developing autoimmune conditions ... but also the endocrine system, and even the child's neurodevelopment.