"A state of complete physical, mental and social well-being, and does not consist only of the absence of disease or infirmity."
In any organism, health is a form of homeostasis. This is a of balance, with inputs and outputs of energy and mass in approximate equilibrium (allowing for growth). Health also implies positive prospects for continued survival.
In humans, with the capacity to analyze and anticipate, health is more than immediate homeostasis. That is, not only must everything be all right at the moment, but there should be subjective understanding that the "healthful" balance will continue. This understanding comes from somatic perception, including pain and discomfort, as well as cognitive perception. In order to feel health, people need to feel that they look well, are functioning as well as they always have, and that no external or internal risk imminently endangers their healthful state.