You cannot research psychological depression without repeatedly stumbling over glib—and always unattributed—references to it as “the common cold of mental illness.”
The only quality that depression shares with that viral annoyance is its pervasiveness. In all other respects, the comparison slanders the nearly 19 million Americans who annually fall prey to depression’s black sorrow, emptiness and devastating hopelessness,  and especially to the more than 20,000 it drives to suicide. 
Its most severe form—major depressive disorder—is the leading cause of disability in the U.S. and other established market economies worldwide. In any given year, it afflicts nearly five percent of the U.S. population over age 18. That translates to 6.7 million women and 3.2 million men. 
In the course of a lifetime, roughly 20 percent of women and 10 percent of men will suffer at least one bout of depression  .
The economic burden of depression is immense. The latest study established the annual cost at $83 billion. That represents $26.1 billion in direct treatment costs, $5.4 billion in suicide related mortality costs and $51.5 billion in lost productivity and other workplace costs. 
What constitutes “depression”?
Let us first be clear on what depression is not: it is not the transient bouts of “the blues” that afflict virtually everyone now and again. Nor is it even applied to the severe sorrow of normal bereavement. Depression is a medical term encompassing three specific conditions – major depressive disorder, dysthymia and bipolar disorder.
Depression is generally defined by a broad range of symptoms and signs, including:
Major depressive disorder is the most profound and disabling form of the illness. Patients have characterized it as being thrown into a dark, profoundly lonely and inescapable pit or dungeon—the embodiment of the motto Dante imagined over the entrance to Hell: “All hope abandon, ye who enter here.” The suicide rate among victims of major depressions is 20 times that of the general population, representing one of every 16 patients diagnosed with the condition. 
While it can occur at any age, the average age of onset for major depressive disorder is the mid-twenties. Untreated, it can last for months or years. Unless they receive maintenance treatment after the initial depressive episode, patients frequently experience one or more additional attacks over the course of their lives.
Dysthymia (dysthymic disorder) differs from major depression largely in its intensity. The patient is functional, but miserable. Since the symptoms are less intense and the signs less visible, sufferers from this degree of depression are frequently undiagnosed. Further restricting the count is the requirement that a diagnosis of dysthymia is appropriate only if the patient has been depressed for at least two years.  It is estimated that only33 to 50 percent of those suffering from depressive illnesses have their conditions properly diagnosed by their primary care physicians. A majority of these undetected cases are probably dysthymia. 
Bipolar disorder, as the name implies, causes the patient’s moods to swing between disabling major depression and mania – a state characterized by extreme energy, little need for sleep, grandiose thinking, irritability, racing thoughts, inappropriate behavior, bad judgment and even delusions. The irrational aspects of mania frequently lead the patient into unpleasant and even dangerous situations. The mood shifts may be quite frequent, and their effects are at least as disabling as major depression. A second form of the illness exists—bipolar II disorder—in which the manic swing is less extreme—a condition called hypomania.
Bipolar disorder affects about 2.3 million adults, or roughly 1.2 percent of the population. Unlike depression, it seems to afflict men and women at about the same rate. It is known to run in families—as does depression. But genetics are considered a much more important factor in the development of bipolar disorder. 
 Strock M, Depression, NIH Publication No. 00-3561, 2000, National Institutes of Mental Health (NIMH); available at http://www.nimh.nih.gov; accessed 30 June 2004.
 Extrapolated from statistics on annual U.S. suicide totals in U.S.A Suicide: 2001 Official Final Data, in Vital Statistics of the United States, Mortality, 2001, National Center for Health Statistics, U.S. Department of Health and Human Services, Washington, D.C., 2003 and the prevalence of major depression among those who complete suicide in Some Facts About Suicide and Depression, American Association of Suicidology (AAS), www.suicidology.org, 2004.
 NIMH, The Numbers Count, NIH Publication 01-4584, 2001; available at http://www.nimh.nih.gov/publicat/numbers.cfm; accessed 30 June 2004.
 Greenberg, PE, et. al., The Economic Burden of Depression in the United States: How Did It Change Between 1990 and 2000? J. of Clin. Psychiatry 2003; 64:1465-1475
 Foregoing list is adapted from AAS, Understanding and Helping the Suicidal Individual, www.suicidology.org, Accessed 6/4/04
 AAS, Some Facts About Suicide and Depression,, available at http://www.suicidology.org, accessed 16 June 2004
 NIMH, The Numbers Count
 Zung WWK, Broadhead WE, Roth ME. Prevalence of depressive symptoms in primary care., J. Fa. Pract., 1993; 37:337-344
 NIMH, The Numbers Count