To say there is a connection between physical illness and depression seems a bit like stating the obvious. Break it down a little and it becomes clear that there are both direct and indirect connections that vary widely. Some diseases and conditions have a clear and direct effect on mood states due to the fact that the nervous system or damage to organs is involved. Others, due to their debilitating and chronic features, and the pain often involved, can result in severe damage to quality of life.
In some conditions and diseases the central nervous system (the brain and spinal chord) is directly affected. Parkinson's disease and multiple sclerosis are two examples. The nature of these illnesses is that nerve damage can directly affect particular areas of the brain that can give rise to depression. Major depression is not inevitable however and estimates vary as to the numbers of people affected. The Multiple Sclerosis Society says that around 50 percent of people with MS will experience "serious depression". In the case of Parkinson's disease, rates of depression, even if not formally diagnosed, are thought to be around 30-40 percent.
Stroke patients who find themselves unable to speak, or who have profound speech difficulties, or have impaired understanding of speech, also suffer with depression. Estimates vary as to the severity of the problem but some research suggests around 30 percent of stroke patients also suffer with depression.
Normal neurotransmitter activity is intimately connected with hormones. A visit to the doctor with complaints of low mood and symptoms of depression is likely to be met with blood tests to establish whether symptoms relate to hormonal imbalance. The thyroid, adrenal and pineal glands can all affect function and induce depression. The adrenal glands secrete cortisol. In situations where too much cortisol is produced (Cushing's disease) various physical and psychological symptoms are seen, including anxiety and depression. Conversely, if too little cortisol is produced (Addison's disease) the symptoms of weakness, lethargy, muscle pains and so on can also indirectly affect mood.
Viral infections such as influenza affect the body's immune system. People who tend to get depressed often have an ineffective immune system. Equally, people with an ineffective immune system also tend to become depressed. It's a vicious circle in which people who pick up an infection like flu often become depressed or despondent after the virus has passed and whilst the immune system repairs. People who already have a low mood at the time of picking up a viral infection are even more susceptible and find their mood state worsens as a result.
Although these issues appear disheartening the situation is perhaps not as bad as it first appears. Depression can stem directly from physical diseases but not as often as might be expected. The highest risk factor for depression during illness is a genetic predisposition to depression. People who were depressed prior to the onset of illness are more likely to develop depression during illness. Conversely, those without a family history of depression and who were never depressed prior to illnesses, have a far higher chance of staying that way even when severely or incurably ill.