Stress (biology) "Physiological stress" redirects here. It is not to be confused with physical stress . Physiological or biological stress is an organism's response to a stressor such as an environmental condition or a stimulus . Stress is a body's method of reacting to a challenge . According to the stressful event, the body's way to respond to stress is by sympathetic nervous system activation which results in the fight-or-flight response . In humans , stress typically describes a n ...