Depression
Depression is the most common of the disorders, with one in five people experiencing bouts each year.
It can happen in short periods of minor upset or for longer periods; and there are a multitude of possible causes.
Those who suffer from depression may lose interest with friends and family, feel hopeless or find difficulty concentrating.
Although depression can be a normal aspect of life, for many it can be a destructive condition, with some even resorting to suicide.
The likelihood of developing the illness is increased if someone has a family history of depression. It can also be caused by events such as losing a job, the end of a relationship, death of a loved one, substance abuse and loneliness.