No one knows exactly what causes a depressive episode. We do know that genetic makeup and events early in life predispose people to develop depression. We also know that stressful life events can precipitate a depressive episode. Some physical illnesses and medications can increase the chances of developing such an episode. We are rapidly learning about the chemical changes that occur in the body when you develop one. All these factors, under the right circumstances, can lead to a depressive disorder.

Upsetting events happen to everyone at some time. Depression often develops when the event is connected with loss of some sort, such as death, divorce, financial losses, job loss, or loss of status. But some people may have characteristics that make them more likely to develop a depressive illness. Your genetic endowment, the quality of your early attachments, your temperament, your sex, and your personality style can all contribute. Even if you have characteristics that predispose you to depression, it will not develop every time there is a loss in your life. However, a depressive episode is six times more common within six months after a stressful life event than at other times.

Temperament
We are born with temperaments that stay with us throughout life. In early infancy, children’s temperaments are characterized by their energy levels, sleeping patterns, eating patterns, and responsiveness to the environment. They can be easy and adaptable, slow to warm up, or difficult to handle. If the parent has difficulty matching the child’s rhythm and temperament, problems can develop in their emotional attachment.

Sex
Major depressive disorder is twice as common in women as it is in men. Both hormone changes and social changes contribute to this difference.

Infections
Viral illnesses often leave someone depressed after the acute illness is over. The reason is not clear, but the phenomenon bis seen in influenza, viral pneumonia, viral hepatitis, infectious mononucleosis, and AIDS.

Autoimmune Diseases
Rheumatoid arthritis and systemic lupus erythematosus are autoimmune diseases that affect many body systems – joints, skin, kidneys, and brain. The underlying problem is an allergic reaction to the body’s own tissue. Depression is often one of the symptoms of these diseases.

Nutritional Deficiencies
Decreased absorption of certain nutrients can show up as depression before other symptoms are present. Both vitamin B12 deficiency and folic acid deficiency can produce depressive illness before the blood counts show anaemia, the symptom that usually alerts the doctor to these deficiencies. The fact that brain changes causing depression may show up first, before the anaemia, has only recently been recognized.

Neurological Illnesses
Many neurological (brain and spinal-cord) illnesses are accompanied by depression that results directly from the illness, not from the person’s distress at being ill. The illness affects the same parts of the brain that are thought to malfunction in depression. Multiple sclerosis, Parkinson’s disease, brain injuries, epilepsy, strokes, and brain tumours can all have depression as part of the picture. In elderly people, it can be difficult to distinguish between the onset of a dementia (such as Alzheimer’s disease) and a reversible, treatable depression.

Metabolic Illnesses
Two very rare metabolic illnesses can also begin with depression – Wilson’s disease and acute intermittent porphyria. Wilson’s disease is an inherited disorder of copper metabolism that leads to a buildup of copper in the body, particularly in the liver, cornea, and brain. Porphyria is an inherited disease caused by lack of a blood enzyme, and symptoms include nausea, vomiting, constipation, muscle weakness, vision problems, and paralysis.

Tumor
Sometimes an underlying brain malignancy will first appear as depression, before the cancer itself is diagnosed.

Medications
Certain medications used to treat medical illnesses can cause depression as a side effect. If you develop depression after you start using blood pressure medication, many forms of steroid medications, anti-inflammatory drugs, and anti-cancer chemotherapy drugs, your doctor may consider the possibility that the depression is a side effect of the medication.