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Stress is a word used in biology and medicine to describe a number of reactions that can be seen in organisms. Stress can be physical or mental. The word 'stress' can describe a pressure or other force, like the pressure of a person's upper teeth on the lower teeth while chewing. 'Stress' can also describe an impact (where one object hits another) that happens more than once. For example, a stress fracture happens when there are repeated impacts on a bone.
More often in medicine, 'stress' describes a living thing's response to a threat or some other change in its environment. This change, or stressor, can be internal (coming from inside the living thing - like an illness), or external (coming from something outside the living thing - like an attack). A living thing's stress response can be physical (with changes happening in the body), mental (with changes happening in the mind), or emotional (with changes happening to a person's or animal's emotions). Stress may also lead to adaptations, which allow certain organisms to better cope with the stressors.
Stress happens every day, and it is part of every living thing's normal life. However, having too much stress can cause many different medical problems. Stress plays a part in some physical problems, like heart disease. Stress also plays a part in many mental illnesses, like anxiety, Acute Stress Disorder, and Post-Traumatic Stress Disorder (PTSD). Acute Stress Disorder and PTSD are mental illnesses that can happen when a person experiences something that is very stressful and terrifying, like a major accident or a war. Another type of stress-related mental disorder is psychophysiologic illness (which used to be called psychosomatic illness). With this type of illness, a person has physical symptoms that are caused by emotional stress, not by an actual physical illness.