An anxiety disorder is a psychological condition marked by excessive worry, fear or panic. It is one of the most common mental illnesses in the world, and can be caused by a wide range of social, behavioral and environmental factors. Stressors may also play an important role in the development of an anxiety disorder. In addition to worry, fear and panic, symptoms may include obsessive or compulsive behaviors, sleeplessness, elevated heart rate, panic attacks and excessive apprehension about past, present or future events. These events may be real or imagined. It is usually treated with a combination of cognitive-behavior therapy (CBT) and anxiolytic drugs which reduce the frequency and severity of symptoms.