Formal depression, which a psychiatrist can diagnose as a mental health disorder, is a serious medical illness that negatively affects how you feel, the way you think and how you act. Major depression strikes about 10% of adults in the United States in a given year, and some 21% are affected during a lifetime. But it’s thought that most people who deal with depression of either type are not diagnosed accurately or aren’t diagnosed as early as they should be. If you feel sad or blue and the feeling persists beyond a few days, and you feel powerless to shake it, it’s time to speak with a healthcare provider.