Stress is the mind's natural response to events that cause us to feel threatened. When confronted by a threat the mind reacts automatically by going into a "fight-or-flight" response. This is a conditioned response.
The "fight or flight" response is the mind's way of protecting you from danger. When this response is working properly it keeps you alert and focused. In emergencies, stress is a life saver giving you enough speed, strength and agility to get yourself out of dangerous situations. A certain level of stress is productive, healthy and necessary.
It's when too much stress is exerted over extended periods that it becomes a problem and starts to work against us. Prolonged stress can lead to a weakened immune system which exposes us to a host of health problems. Prolonged stress can also lead to higher blood pressure which in turn can lead to heart attacks and strokes.