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, but 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.
Anxiety is another widespread disorder, affecting almost 5% of people in the UK.
While it is perfectly normal to feel anxious about certain situations, such as starting a new job or school, or moving away from home, anxiety can affect people in more serious ways.
Feelings of anxiety cause the body to release hormones, which can increase the heart rate and lead to sweating. It can also affect a person’s everyday life by lowering self-esteem, avoiding social situations and preventing them from even leaving their house.
There are 10 types of personality disorders, including schizoid and narcissistic, with around 4% of the UK being affected by at least one of them.
A person suffering from a personality disorder will experience feelings and emotions which are far different to people around them, such as anxiety and paranoia.
For example, people who have developed a paranoid personality disorder will experience severe feelings of distrust towards others; even close friends and family. They may also assume people are constantly looking at or judging them, and will constantly be hunting for validation of their fears.
Post traumatic stress disorder is triggered by a specific event, and has symptoms including anxiety, flashbacks of the event itself and feeling jumpy or on edge.
The causes of this disorder are numerous and extremely varied. Examples include sexual abuse, road accidents, military combat, natural disasters and being held hostage.
It is estimated that of all people who have had a traumatic experience, around one in three will develop post traumatic stress disorder.