For some people, a combination of many factors may be causing clinical depression. For others, a single factor may be triggering the illness. Depression often is related to the following.

Imbalance of brain chemicals called neurotransmitters - Changes in these brain chemicals may cause or contribute to depression.

Certain diseases or illnesses - Ailments such as cancer, heart disease, Parkinson's disease, Alzheimer's disease, diabetes and hormonal disorders can often trigger clinical depression. This is referred to as co-occurring depression.

Negative thinking patterns - People who are pessimistic, have low self-esteem, worry excessively, or feel they have little control over life events are more likely to develop depression.

Family history of depression - A genetic history of clinical depression can increase one's risk for developing the illness. But depression also occurs in people who have had no family members with depression.

Difficult life events - Events such as the death of a loved one, divorce, financial strains, history of trauma, moving to a new location or significant loss can contribute to the onset of clinical depression.

Certain medications - Some medication can actually cause clinical depression. Therefore, it is important that people inform their doctors of all medications they are taking and report any depressive symptoms.

Frequent and excessive alcohol consumption - Drinking large amounts of alcohol on a regular basis can sometimes lead to clinical depression.