Medically, one can distinguish between several forms of appearance of depressions:

Major depression (MD) is characterized by a combination of symptoms that severely derogate the ability to work, study, sleep, eat and enjoy pleasurable activities. MD can be a singular incidence like a bout or, more frequently, preservative.

A less severe type of depression, dysthymia,features long-term, chronic symptoms restraining well-being. Dysthymia may pass into MD.

Bipolar disorder (BD) also called manic-depressive illness is characterized by cycling mood changes: severe highs (mania) and lows (depression). In the depressive phase, one can show any or all symptoms of a depressive disorder. The manic phase accompanies hyper-activity. Mania often gravely constricts thinking, judgment and social behavior. Untreated mania may lead to a psychotic state.

In the recent past, researchers have shown that physical changes can be accompanied by mental changes as well. (Compare: Juvenal: "Orandum est, ut sit mens sana in corpore sano". It is to be prayed that the mind be sound in a sound body.)
Diseases like strokes, heart attacks, cancer, Parkinson's disease and hormonal disorders can cause depression, leading, on its part, to apathy and, thus, prolonging the recovery period. Also, a tragic loss, relationship problems, money troubles or chronic stress can trigger a depressive attack. Very often, a combination of genetic, psychological and environmental factors is responsible for the onset of a depressive disorder. Later disease stages typically are precipitated by only mild or absent stresses.

For further information visit the National Institute of Mental Health.