Good bacteria are even present in—and good for—infants. In fact, recent studies show that a young baby may not develop a healthy mix of “good” intestinal bacteria if they are delivered by Caesarean section or don’t drink breastmilk. The scientists believe certain microbes in the gut are linked to a healthy digestive tract and also help to stimulate the immune system. These microbes apparently signal the immune cells to not overreact to certain bacteria and react to others. And if the bacteria are not present, the immune system seems to overreact to something benign, say, dust or food—developing certain medical problems, such as asthma and allergies to foods.