This psychological disorder affects people by causing sadness, worthlessness, insomnia, difficulty concentrating, etc. Of the millions of people affected by depression, only a third eventually seek treatment. For those who don't seek help, depression can get worse and last longer. There are multiple forms of this disorder, such as major depression, dysthmic disorder, psychotic depression, postpartum depression, seasonal affective disorder, and bipolar disorder (a manic-depressive disorder).