Depression.

Depression is one of the most common physiological problems and almost everyone is affected by it, whether it is themselves or from friends or family. Depression effects and usually causes problems with work, friends, and most importantly, family and loved ones. Depression causes horrible emotional pain, influences your will to do the stuff you "used" to love, and has a negative impact on the economy, so bad as 44 billion dollars. Depression is a psychological condition that changes how you think and feel, and also affects your social behavior and sense of physical well-being.
 Nearly two-thirds of depressed people do not get the proper treatment and their symptoms are not recognized as depression, they are seen as weak or lazy, the people effected sometimes are unable to even reach out for help, or individual symptoms are treated and not the underlying cause.
There is also such this as bipolar depression. The main difference between the two is that bipolar people suffer from extreme mood swings. Bipolar disorder usually begins in late childhood and stays with you through adulthood. It is often overlooked because it is episodic. Many people with bipolar disorder also lose their jobs, relationships, money, and sometimes their life because they don't get the proper treatment. Many of these people also end up abusing drugs and alcohol.
This is really sad because over 80 percent of people with depression getting the proper treatment make great improvements in their life.