About 1 in 10 Americans suffers some form of depression, according to the National Institute of Mental Health. Women experience depression twice as often as men. Depression has devastating effects on a person's relationships with family and friends, on the ability to do productive work, and, of course, on the ability to enjoy life.

People often say "I'm depressed" when they feel blue, down, sad, gloomy, cheerless or just plain world-weary. But for doctors and mental health professionals, depression is a condition with well-defined symptoms. Sadness is just one. Other symptoms include:

When professionals use the word "depression," they're referring to an illness that deserves care and treatment. Some depressive disorders are mild, others severe. Depression can occur even when the person and family members can’t think of a good reason why. Sometimes there is a trigger, such as a troubled family relationship, career disappointment, or death of a loved one. Genetics also play a role, although the specifics are poorly understood.

Depression can be associated with other medical conditions. Health-care professionals see depression more often in people who have serious health problems, such as heart disease and cancer. Depression also can occur in connection with illnesses that affect the brain, such as Parkinson’s disease and stroke.

Fortunately, depression can be treated. Drug treatment is helpful, and so is psychotherapy (talk therapy).