The concept of health and disease, in the minds of most people and many doctors, is simple and clear cut. A person is healthy if he has no disease. Diseases are well defined and clearly recognizable conditions.
The reality is not so. Health is not the same in all so called healthy people. It covers a wide spectrum of conditions physically, mentally, and socially. Some people are physically robust, happy, psychologically satisfied, and socially integrated and active. Others are frail, easily tired, worried, frequently unhappy and socially isolated. And there are all grades in between. That is why the aim of medicine is not only to prevent or treat disease but also to promote health.
Apart from acute conditions like acute infections and trauma, the transition between health and disease is gradual and ill defined. Most symptoms of disease are present to some degree in so called normal people. All people become short of breath if they exercise sufficiently and "sufficiently" varies in various people. Some may need to climb fifty stairs to become short of breath. Others may become so after climbing thirty or twenty stairs and so on. People with heart or lung disease may become short of breath on climbing ten stairs or one or two depending on the severity of their illness. What is the number of stairs that makes shortness of breath pathological? Can we put a definite figure? You can say the same about poor appetite, headache, anxiety, numbness, poor memory, readiness to fall asleep on going to bed, muscle and joint pains and so on. Recognizing a symptom as abnormal or pathological varies widely between people. That is one of the reasons why some people go to the doctor frequently and sometimes for no good reason, while others do not go unless they become severely ill.
A similar thing can be said about laboratory tests, but in this case artificial, statistically derived, boundaries have been created by putting a reference range for every test. These reference ranges have come to be generally conceived as the boundaries between normal and abnormal test results. According to these boundaries, a test result can deviate from the average of normal people to a large extent and still be considered normal. Then when it crosses the artificially drawn boundary it suddenly comes to be regarded abnormal!! The transfer from health to disease is not that sharp in real life.
The spectrum of health is wide and so is that of disease and the border between the two is frequently blurred and difficult to discern.