The "hygiene hypothesis" proposes that past generations were exposed to greater numbers and varieties of microbes, which may have helped humans develop strong immune systems. Our reduced exposure to those germs now may be increasing the risk of immune disorders, including allergies. Support for the idea comes from studies that have linked attending day care or growing up around pets, farm animals, or older siblings to a reduced risk of allergies.
While those findings are still preliminary, they do suggest that overly strict hygienic standards may not be helpful. For example, while it pays to be vigilant when there's a strong risk of infection, such as before and after handling food, "it's clearly not harmful for kids to play in the dirt, and not necessary for them to wash their hands 17 times a day," says Joel Weinstock, M.D., an immunologist and professor of medicine at Tufts University in Boston. The evidence may also provide another reason to avoid antibacterial soap and cleaners, which may contribute to the spread of antibiotic-resistant bacteria.
