One disorder which largely goes undiagnosed and untreated is Depression.
When a person is feeling sad and is in a low mood we normally say that he is feeling depressed but then we can't run to the doctor and ask for a treatment because somebody is feeling sad and blue! It is only when that person is persistently feeling the same way and a result of which his or her life is being affected does it require some attention.
Medically depression is considered as the imbalance of the brain chemicals responsible for carrying signals to brain and nerves called as neurotransmitters.
So when the symptoms mentioned below are noticed in anybody know then that it should raise an alarm in your head and you should do your best to make the person visit a psychiatrist.
Although the symptoms vary person to person, there are some which may be called as quite common.
They are:

• Constant feeling of sadness
• Feeling worthless and guilty
• Almost always feeling irritable
• Loss of energy even though physically the person may be fit and is hardly involved in any physical activity.
  

• Change in sleeping pattern - either he/she cannot or finds it hard to sleep or perhaps is sleeping a little too much.
  
• Change in eating habits -loss of appetite or in some cases overeating
• Constant headaches and digestive problems even when being treated for the same.
  
• Constantly talks of suicide and death

The following may be considered as causes for the occurrence of the above symptoms:

1. If the person has a family history of depression
2. If he/she is already suffering from post traumatic stress or compulsive-obsessive disorder, social phobia or other such anxiety disorders which are known to precede depression
3. If there has been a breakup in a close knit relationship like marriage
4. If the person is generally one with a pessimistic attitude.
   
5. In the case of a woman it can be the hormonal changes during puberty, menstruation, menopause and pregnancy.
   
6. In case of elders it may be illness or not being able to be physically active.
   

Depression can ruin one's life.
It can affect his work and his relationships, his overall health may be adversely affected.