In the United States alone, it is estimated that severe depression affects more than 15 million people. Depression is so widespread that it is sometimes called “the common cold of mental illness.” It is also the most treatable of mental disorders, with up to 80 percent of patients showing marked improvement after therapy. Nonetheless, only about 30 percent of depression sufferers seek professional help. Those who don't often feel the condition carries a stigma. The pain endured by untreated or misdiagnosed depression sufferers is also felt by society at large. According to the government, depression costs the nation some $27 billion a year, chiefly in reduced productivity and lost work time. Despite the problems, researchers remain cautiously confident of further progress in understanding and treating the debilitating affliction.