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According to the so-called hygiene hypothesis, spending over 90 percent of our time in the bacteria-poor environment indoors, as we do (especially early in life, when our immune systems are being formed), can cause our bodies to overreact to harmless substances later on, making us sick.
“Allergies and asthma are both examples of the way that the immune system is misfiring,” said Jordan Peccia, a professor of environmental engineering at Yale University. “An allergy is our immune system attacking something that it shouldn’t attack, because it hasn’t been calibrated properly.”
Dr. Peccia said exposure to animal micro-organisms during the first three months of life helps to stimulate a child’s immune system so that it doesn’t become overly sensitive later in life. A study published last year in The New England Journal of Medicine found that Amish children in Indiana who grew up close to barnyard animals had far lower rates of asthma than Hutterite children, who were raised apart from animals on large mechanized farms in North Dakota.
A co-author of the study, Jack Gilbert, the director of the Microbiome Center at the University of Chicago, said that the Amish suffer from fewer immune-related illnesses than the rest of us because they grow up with their livestock and the bacteria they host, as our human predecessors did for thousands of years.
