Stress (biology)
 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. 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. With this type of illness, a person has physical symptoms that are caused by emotional stress, not by an actual physical illness.