DEPRESSION HAS PROBABLY been around as long as the human race. In classical Greece and medieval Europe it was termed "melancholia," one of the four fluids people were believed to have in their bodies that determined their personality traits. These unfortunate souls probably received no treatment and could be expected to remain "melancholic" for life.

Throughout literary history, writers have documented their depression in autobiographies, journals, fiction, and poetry. But it is only in recent years that depression has been understood as a serious—and treatable—disease. The biology of depression is just now being examined for the first time, and each year brings startling new findings. Researchers are discovering how subtle changes in brain chemistry can have profound effects on mood and behavior. Effective new treatments are being tested and approved for use.

An underdiagnosed and misdiagnosed disease

Still, depression may...