Everybody feels down for a day or two. But one in 14 American adults—15 million in all—has clinical depression each year.

Depression is a complex condition characterized by profound sadness, lethargy, feelings of worthlessness, and a loss of interest in social activities. These feelings can last for two weeks or go on for decades. The difference between the blues and clinical depression is one of length and severity of symptoms.

“If you’ve gotten to the point where you’re looking up depression on the computer, then there’s a good chance it’s a real depression, not the blues,” says Tracey Lipsig Kite, MSW, a licensed therapist in private practice in Evanston, Ill. “If you really are to the point where you think you might need therapy, you’re probably right.”