Health is the level of functional or metabolic efficiency of a living organism. In humans, it is the ability of individuals or communities to adapt and self-manage when facing physical, mental, or social challenges. The most widely accepted definition of good health is that of the World Health Organization Constitution. It states: "health is a state of complete physical, mental and social well-being and is not merely the absence of disease or infirmity" (World Health Organization, 1946). In more recent years, this statement has been amplified to include the ability to lead a "socially and economically productive life." The WHO definition is not without criticism; mainly that it is much too broad.