http://www.nimh.nih.gov/health/topics/generalized-anxiety-disorder-gad/index.shtml
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National Institute of Mental Health, National Institutes of Health
Generalized Anxiety Disorder (GAD) is a real illness. GAD can be treated with medicine and therapy. If you have GAD, you worry all the time about your family, health, or work, even when there are no signs of trouble. Sometimes you aren't worried about anything special, but feel tense and worried all day long. You also have aches and pains for no reason and feel tired a lot. Everyone gets worried sometimes, but if you have GAD, you stay worried, fear the worst will happen, and cannot relax.