Why health do not consists an absence of disease or infirmity?

World Health Organization's definition of Health is a state of complete mental, physical and social well-being and not merely the absence of a disease. "Not merely the absence of a disease" meant that a person who does not have any symptom of disease or is not diagnosed with a disease, is healthy. For a person to be stated as healthy, he must be of sound mental (no apparent signs or symptoms of deviation), physical (no objective and subjective signs and symptoms of disease or illness) and social well-being (can interact, can communicate normally).