The mood disorder may descend seemingly out of the blue, or it may come on the heels of a defeat or personal loss, producing persistent feelings of sadness, worthlessness, hopelessness, helplessness, pessimism, or guilt. Depression also interferes with concentration, motivation, and other aspects of everyday functioning.
Depression is a complex disorder, involving many systems of the body, including the immune system, either as cause or effect. It disrupts sleep and it interferes with appetite; in some cases, it causes weight loss; in others, it contributes to weight gain.
Because of its complexity, a full understanding of depression has been elusive. But depression involves mood and thoughts as well as the body, and it causes pain for both those living with the disorder and those who care about them. Even in the most severe cases, depression is highly treatable. The condition is often cyclical, and early treatment may prevent or forestall recurrent episodes.