The "fight-or-flight" response refers to the body's natural reaction to an emergency. The body systems which are useful for facing a threat ("fight") or fleeing danger ("flight") are stimulated: blood flow is increased to the muscles, heart rate increases, respiration increases, and blood pressure increases. As a tradeoff, energy is decreased to other ongoing body functions including digestion, immune system response, ovulation, and so on.

In the short term, the fight-or-flight response is a natural and appropriate response to stress or threat. In fact, it is generally believed that a certain amount of stress is necessary for physical well-being. However, prolonged stress can damage body function, leading to high blood pressure, gastrointestinal and digestive problems, decreased immune system response, and even brain damage.