Clinical depression is characterised by overwhelming prolonged negativity, fatigue, apathy and feelings of hopelessness.
People with depression may be unable to concentrate or go to work, or they may experience breakdown in relationships and contemplate suicide. They may be unable to find pleasure in anything.
Depression is poorly understood. It can be triggered by emotional trauma, but some people seem to be genetically predisposed.
On average, one in five people will experience it during their lifetime. A combination of counselling and antidepressant medication is the most common treatment.
Recent research has shown that certain antidepressants do not just regulate neurotransmitter levels but may also work by increasing nerve connections in parts of the brain used for thinking and feeling.