Anxiety, on the other hand, is more general and complex. It is felt in anticipation of danger, and is associated with the ability to predict, prepare for, and adapt to change. Often, it lasts a long time, and its cause remains ill-defined. For example, someone uneasy about public speaking may experience a tightness in the stomach for days before a scheduled talk.
As long as there's a good reason for fear or anxiety, and it doesn't interfere with the ability to work, play, and socialize, it is not considered a problem. But when anxiety takes on a life of its own and begins to disrupt everyday activities, the situation is no longer normal. A genuine emotional disorder is now at work... and it's time to see a doctor.
Major depression, the kind of depression that will most likely benefit from treatment with medications, is more than just "the blues." It is a condition that lasts 2 weeks or more, and interferes with a person's ability to carry on daily tasks and enjoy activities that previously brought pleasure. Depression is associated with abnormal functioning of the brain. An interaction between genetic tendency and life history appears to determine a person's chance of becoming depressed. Episodes of depression may be triggered by stress, difficult life events, side effects of medications, or medication/substance withdrawal, or even viral infections that can affect the brain.