Stress refers to an organism's failure to respond appropriately to threats, whether real or imagined. Common external stressors include work, life events, relationships, finances, and family. Internal stressors include inability to cope with change, unrealistic goals, and low self-confidence. Major life events like death, illness, marriage and job changes are among the top ten commonly accepted stressors. Physical symptoms of stress include illness, headaches, and effects on the brain and immune system. Hans Selye discovered the biological stress response and stress syndrome through his research. Stress can be neutral, negative, or positive, and everyone experiences it at some point.