Stress is a condition of the mind and a factor in the expression of disease that differs among individuals. In post-traumatic stress disorder (PTSD), traumatic events can create a long-lasting state of physiologic reactivity that amplifies and exacerbates the effects of daily life events. The elevated activities of physiologic systems lead to wear and tear, called "allostatic load." It reflects not only the impact of life experiences but also of genes, individual life-style habits (e.g., diet, exercise, and substance abuse), and developmental experiences that set life-long patterns of behavior and physiologic reactivity. Hormones associated with stress and allostatic load protect the body in the short run and promote adaptation, but in the long run allostatic load causes changes in the body that lead to disease.