Over attention to the biology of disease has led to a kind of tunnel vision which sees all disease as caused by one "bug" or another, and one chemical imbalance or another. Therefore, the solution to disease is seen as taking physical actions to correct the chemical imbalance or to kill the guilty bug in question.
To gain a broader perspective of disease we need to develop an understanding of the psychology of disease, not just its physical manifestation. For, although physical factors such as diet and exposure to harmful substances can be important, we can’t forget that our mind is, to a large extent, the determining factor of how our biology operates. It controls- or strongly impacts upon- our hormone levels, the effectiveness of our immune system and many other internal processes that we are not consciously aware of!
Without paying attention to our thoughts and their effects on our emotions and biology we may be missing a large piece of the health puzzle! To truly understand disease and avoid it we need to pay attention to our mental processes and their physical effects in addition to what we expose our body to physically. In this way we maximise our chances of leading a healthy life!
See also: Mind and matter are interweaving processes- SETH