Cortisol is a hormone secreted in the adrenal gland. It helps the body respond to stress. During stress, the body's "fight-or-flight" response makes sure that plenty of energy is available to systems like the heart and muscles by temporarily decreasing energy to other ongoing functions in the digestive system, the immune system, the reproductive system, and so on. In the short term, this is natural and desirable. However, prolonged stress can lead to overproduction of cortisol which can cause long-term suppression of the immune system, making the body more vulnerable to infection.