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Stress is a word used in medicine to describe many different things. 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 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.