Can kids be “too clean”? How often and how rigorously children should wash is not as clear-cut as you might think. If your tot’s face and hands are smeared with jelly or his hair is a tangled mass, a douse of warm water is a good idea—but beyond that, what is the right level of hygiene for kids?

One school of thought, called the “hygiene hypothesis,” suggests that a lack of early childhood exposure to infectious agents and microorganisms increases susceptibility to illness by changing how the immune system reacts to germs. This theory, first developed in the late 1980s, is not without detractors, but recent research, including a 2009 study conducted at the University of California, San Diego, suggests that America’s overly clean, “antibacterial” lifestyle may be weakening children’s defenses against allergens and inflammation. “Germs are not the enemy,” insists Lawrence B. Palevsky, MD, cofounder and president of the Holistic Pediatric Association. “Bacteria on skin play an important role in keeping us healthy.”