Stress is the release of neurochemicals and hormones being released in the body; as a result, your "fight or flight" response comes into play. It's because of stress that we prepare ourselves for any complex situation that may come to pass. Your blood rate begins to increase, the adrenaline begins to kick in, and you focus begins to enhance. This may be fine at times, however, there can be some negative consequences if the issue is prolonged.  If the proper action is not taken, the stress response would generate health issues.  There are a number of physical illnesses that could occur such as heart disease or high blood pressure. Same goes mental illnesses like anxiety or depression. Despite this, stress doesn't necessarily always qualifies as a bad thing. It is also our reason for survival.