Fear can be bad if it's chronic. If someone is afraid or anxious all the time it can have negative effects on the body. Too much fear in one moment can cause someone to freeze up and fail to react.

But if you can discipline it, fear can be useful. When you're afraid, the body responds by releasing chemicals that increase the function of the sympathetic nervous system and the adrenal-cortical system. This has the effect of preparing your body to deal with an immediate threat. Adrenaline is released into the bloodstream. Heart rate, blood pressure and blood glucose levels rise. Muscles tense up in preparation and the lungs take in more oxygen. Non-essential systems like digestion temporarily shut down to allow more energy for emergency functions. And the brain focuses on the immediate danger.