According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR), depression occurs when you have at least five of these symptoms at the same time for at least two weeks: depressed mood during most of the day, fatigue or loss of energy, feelings of worthless or guilt, impaired concentration, insomnia or hypersomnia, markedly diminished interest or pleasure in almost all activities, recurring thoughts of death or suicide, sense of restlessness or being slowed down, significant weight loss or gain.  
However, there is no single known cause of depression. Rather, it likely results from a combination of genetic, biochemical, environmental and psychological factors. Depression could be difficult to diagnose, because sometimes people fail to realize or accept that there is a physical cause. Physical symptoms of depression are: headaches, neck aches, dizziness, chest pain, rapid heartbeat, digestive problems, stomach pain, appetite changes, muscle aches, muscle spasms and back pain. Why patients have physical symptoms? Because the brain chemicals or neurotransmitters, serotonin and norepinephrine, influence mood and pain. According to the National Institute of Mental Health in United States, about 14.8 million adults suffer from major depression. Unfortunately, most people with clinical depression never seek treatment. Suicide is the third leading cause of death in people between ages of 10 and 24. Depression can get worst if it is untreated and could end it in suicide. Warnings of suicide are: thoughts or talk of self harm or harm to others, aggressive behavior or impulsiveness, previous suicide attempts can increase the risk for future attempts.
When I hear the word depression I think in a big umbrella, because there are different types of depression: major depression – affects about 6.7% of the US population over age 18. It could be undiagnosed and untreated in children and the elderly. Also, about twice as many women as men have major depression....