Stress is what you feel when you have to handle more than you are used to. When you are stressed, your body responds as though you are in danger. It makes hormones that speed up your heart, make you breathe faster, and give you a burst of energy. Some stress is normal and even useful. It can help if you need to work hard or react quickly. But if stress happens too often or lasts too long, it can have bad effects. Stress can be linked to headaches, upset stomach, back pain and trouble sleeping. It can weaken your immune system making it harder to fight off disease. If you already have a health problem, stress may make it worse. The good news is that you can learn ways to manage stress.