Sadness or depression may be caused by a setback or a loss, and anxiety may be triggered by a threat or a challenge. Often these feelings of depression or anxiety are appropriate and transitory. But if they persist or occur spontaneously and begin to impair daily life, a mood or anxiety disorder may be present.
Mood disorders include depression and bipolar disorder (a term used to indicate an illness with episodes of both depression and mania). Usually, both depression and bipolar disorder are episodic—that is, bouts of illness are separated by periods of remission.
Anxiety disorders are characterized by either recurrent or persistent psychological and physical symptoms that interfere with normal functioning, continue in the absence of obvious external stresses, or are excessive responses to these stresses. Specific anxiety disorders include generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, and phobic disorders.