Health is the level of functional and/or metabolic efficiency of an organism at both the micro (cellular) and macro (social) level. In the medical field, health is commonly defined as an organism's ability to efficiently respond to challenges (stressors) and effectively restore and sustain a "state of balance," known as homeostasis.
Another widely accepted definition of health is that of the World Health Organization "WHO". It states that "health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity". In more recent years, this statement has been modified to include the ability to lead a "socially and economically productive life." The WHO definition is not without criticism, as some argue that health cannot be defined as a state at all, but must be seen as a process of continuous adjustment to the changing demands of living and of the changing meanings we give to life. The WHO definition is therefore considered by many as an idealistic goal rather than a realistic proposition.