In the fields of psychology and psychiatry, the terms depression or depressed refer to sadness and other related emotions and behaviors. It can be thought of as either a disease or a syndrome.

Biological influences of depression are varied, but may relate to malnutrition, heredity, hormones, seasons, stress, illness, drug or alcohol use, neurotransmitter malfunction, long-term exposure to dampness and mold[1], back injury, and to aerosol exposure. There are also correlations between long term sleep difficulties and depression. Up to 90% of patients with depression are found to have sleep difficulties.

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How Do People Respond to Someone Who's Depressed?
Sometimes friends or family members recognize that someone is depressed. They may respond with love, kindness, or support, hoping that the sadness will soon pass. They may offer to listen if the person wants to talk. If the depressed feeling doesn't pass with a little time, friends or loved ones may encourage the person to get help from a doctor, therapist, or counselor.
But not everyone recognizes depression when it happens to someone they know.
Some people don't really understand about depression. For example, they may react to a depressed person's low energy with criticism, yelling at the person for acting lazy or not trying harder. Some people mistakenly believe that depression is just an attitude or a mood that a person can shake off. It's not that easy.
Sometimes even people who are depressed don't take their condition seriously enough. Some people feel that they are weak in some way because they are depressed. This is wrong — and it can even be harmful if it causes people to hide their depression and avoid getting help.
Occasionally, when depression causes physical symptoms (things like headaches or other stress-related problems), a person may see a doctor. Once in a while, even a well-meaning doctor may not realize a person is depressed, and just treat the physical symptoms.

sources: http://kidshealth.org/teen/your_mind/mental_health/depression.html
http://www.channelnewsasia.com/stories/health/view/441617/1/.html

amanda goh, 211 (2)