Why are allergies becoming so common? Some scientists theorize that our lives aren’t dirty enough. Hygiene, says one researcher, is the major health change of the past 200 years, making unsanitary crowded environments and the associated bacteria, viruses, and parasites—for the most part—things of the past. That may explain some compelling findings: Respiratory allergies appear to be less prevalent in poor societies than in rich ones. People raised on farms have lower allergy rates than those who grew up in cities. And children who attended day care have lower rates of allergies. So, maybe Pigpen has the right idea.