Fortunately, depression is treatable. In the past 18 years, the Food and Drug Administration has approved several new antidepressants, including Wellbutrin (bupropion), Prozac (fluoxetine), Zoloft (sertraline), Paxil (paroxetine), Effexor (venlafaxine), Serzone (nefazodone), and Remeron (mirtazapine). According to the American Psychiatric Association (APA), 80 to 90 percent of all cases can be treated effectively. However, two-thirds of the people suffering from depression don’t get the help they need, according to NIMH. Many fail to identify their symptoms or attribute them to lack of sleep or a poor diet, the APA says, while others are just too fatigued or ashamed to seek help. Left untreated, depression can result in years of needless pain for both the depressed person and his or her family. And depression costs the United States an estimated $43 billion a year, due in large part to absenteeism from work, lost productivity, and medical costs, according to the National Depressive and Manic Depressive Association.

The three main categories of depression are major depression, dysthymia, and bipolar depression (sometimes referred to as manic depression).

Major depression affects 15 percent of Americans at one point during their lives, according to the U.S. Department of Health and Human Services. Its effects can be so intense that things like eating, sleeping, or just getting out of bed become almost impossible. Major depression “tends to be a chronic, recurring illness,” Laughren explains. Although an individual episode may be treatable, “the majority of people who meet criteria for major depression end up having additional episodes in their lifetime.”

Unlike major depression, dysthymia doesn’t strike in episodes, but is instead characterized by milder, persistent symptoms that may last for years. Although it usually doesn’t interfere with everyday tasks, victims rarely feel like they are functioning at their full capacity. According to the National Alliance for the Mentally Ill, almost 10 million Americans may experience dysthymia each year.

Finally, bipolar disorder cycles between episodes of major depression and highs known as mania. Bipolar disorder is much less common than the other types, afflicting about 1 percent of the U.S. population. Symptoms of mania include irritability, an abnormally elevated mood with a decreased need for sleep, an exaggerated belief in one’s own ability, excessive talking, and impulsive and often dangerous behavior.

Study after study suggests biochemical and genetic links to depression. A considerable amount of evidence supports the view that depressed people have imbalances in the brain’s neurotransmitters, the chemicals that allow communication between nerve cells. Serotonin and norepinephrine are two neurotransmitters whose low levels are thought to play an especially important role. The fact that women have naturally lower serotonin levels than men may contribute to women’s greater tendency to depression. Family histories show a recurrence of depression from generation to generation. Studies of identical twins confirm that depression and genes are related, finding that if one twin of an identical pair suffers from depression, the other has a 70 percent chance of developing the disease. For fraternal twins or siblings, the rate is just 25 percent.