In a course I took in summer (the first time in a long, long time I was interested in a course of study), I learned that while past definitions have emphasised merely the absence of disease or illness, modern definitions view health as more holistic, involving overall physical, psychological, emotional and social state of well-being. The World Health Organisation definition of 1948, that health is "a complete state of physical, mental and social well-being and not merely the absence of disease and infirmity", is a good example.
Disease, an unpleasant experience with outcomes ranging from discomfort to loss of life, is something dreaded, something to be prevented at all costs. Of course, nobody welcomes disease, but according to a Chinese article forwarded to me by a friend, disease serves an important purpose. Through instigating pain in the body, disease tries to correct our mistakes. The pain means we need to change.
I think this is very true. Look at how many diseases, or even deaths, are absolutely preventable. It is amazing how a change of lifestyle or diet can help prevent or, in some cases, revert diseases. It is even more amazing, however, how most people resist the change even though they know they will pay a heavy price or have already done so. Even the painful or even fatal lesson that disease tries to teach is not enough to make people change. Such is the appeal of the unhealthy diet and lifestyle.