Professor Martin Prince, professor of epidemiological psychiatry at King's College, London has tried to calculate in financial terms how much of a burden a depressed person can become.

BBC: Depression looms as global crisis

Dr. Hyman says there's reason to believe that the stereotype of the depressed person being someone who is withdrawn and inactive may not always be correct, especially among adolescents.

CNN: Research on depression shows genetic and chemical link

One thing we know for sure is that the longer a person is depressed, the less likely he or she is to recover and the less likely to respond to treatment.

CNN: How can I help my despairing son?

Almost all episodes of depression are set off by one or the other of these factors, although the more often a person is depressed the less adversity is needed to set off a new episode.

CNN: Enke death: What are the roots of depression?

It is not difficult to claim otherwise when a person says they are depressed.

FORBES: Fraud And Disability Equal A Multibillion Dollar Black Hole For Taxpayers

The question of why severely depressed patients suffer from infections more often than the average person attracted the attention of Gregory Miller, a psychologist at Carnegie Mellon University, also in Pittsburgh.

ECONOMIST: The power of negative thinking