Usually these last for only one to several hours or days. Some people experience more prolonged periods of sadness following the loss of a friend or family member. This is usually described as grief or bereavement. A depressive illness is a persistent lowering of mood lasting for several weeks at a time, and accompanied by a specific group of physical and psychological symptoms. This may cause serious physical, psychological and social problems, and requires specific treatment.
Depression is a very common illness.
Many are unaware of the cause of their physical and psychological distress and do not receive treatment for it. Many different types of depressive illness exist. Often depression develops in response to illness, stress or losses of various types. The other major type of depression, endogenous depression, occurs for no obvious cause, and is due to changes in various brain chemicals, the neuro-transmitters. Sometimes people with this biological type of depression also experience periods of elevated mood, or highs.