Health can be defined negatively, as the absence of illness, functionally as the ability to cope with everyday activities, or positively, as fitness and well-being (Blaxter 1990). In any organism, health is a form of homeostasis. This is a state of balance, with inputs and outputs of energy and matter in equilibrium (allowing for growth). Health also implies good prospects for continued survival. In sentient creatures such as humans, health is a broader concept.

The World Health Organization defines health as "a state of complete physical, mental and social well-being, and does not consist only of the absence of disease or infirmity." This is a useful, but very idealistic and non-realistic definition. It classifies 70 to 95 percent of all people as unhealthy.  » More