Stress affects our bodies in ways that we do not yet understand. Scientists know that many types of stress activate the body’s endocrine (hormone) system, which in turn can cause changes in the immune system, the body’s defense against infection and disease (including cancer). On the positive side for women, there is some evidence that women who breast-feed their infants produce lower levels of stress response hormones, such as adrenalin and cortisol, than do women who bottle-feed.