Without exposure to germs, we now know, a child’s immune system is prone to dysfunction. In 1989, the immunologist David Strachan proposed that having older siblings, belonging to a large family, and living in an environment that was not overly sanitized might help protect children from developing asthma and allergies. This “hygiene hypothesis” suggested that it was possible to be too clean and too free of disease. As the hygiene hypothesis took hold, scientists searched for one particular childhood disease that might prevent allergies, but this thinking gave way to the understanding that the
...more
