That is correct – our immune systems were designed to cope with a world full of germs. Unless you live on a farm, postindustrial life can be relatively sterile. According to the hygiene hypothesis, a lack of exposure tips the immune system toward inflammation and allergic tendencies, as does the use of antibiotics in the first year of life. In addition, children born by caesarean section face a higher risk of allergies and asthma, because passage through the birth canal inoculates infants with bacteria that normally populate skin, the upper respiratory tract, guts and other organs. The development of healthy gut bacteria positively shapes the immune system.

Proponents of the hypothesis point out that kids who attend day care early in life, grow up in larger families, or spend time around barnyard animals (or at least dogs) are less likely to develop asthma, hay fever and eczema. Critics of the hygiene hypothesis respond that asthma rates have also soared in recent years among children in cities, who may also be exposed to different kinds of dirt and germs. Other possible factors contributing to higher allergy rates in the United States include increased consumption of junk food, inactivity and obesity.

We can all agree on the benefits of clean drinking water and modern sanitation methods. Meanwhile, here’s how you can expose yourself and your family to reasonable levels of germs: Spend time outdoors. Garden. Play with a dog. Afterward, wash up with plain soap and water. If you already have asthma aggravated by dust mites, reduce symptoms by keeping a clean house and enclosing pillows and mattresses in airtight covers.