Tough personal experiences can trigger depression, or it may develop out of the blue.

Of course, anyone who gets a serious illness is vulnerable to depression, and a disease like cancer can itself trigger depression.

If this is true for you, a stressful life event is more likely to trigger depression.

Sometimes even happy life events, such as a marriage or promotion, can trigger depression because of the stress that comes with change.

An example of this is a bad practical joke played on someone which triggered serious depression in that person.

The period of polar night can trigger depression in some people.

Sometimes, external events a death in the family, say can trigger debilitating depression or anxiety.

Yet even positive changes - like getting married or starting a new job - can sometimes trigger depression.

"Typically by the time that a couple comes into treatment this stress has triggered anxiety or depression in one or both of them."

Some medicines can trigger depression, such as steroids or narcotics for pain relief.