Health is a term that refers to a combination of the absence of illness, the ability to cope with everyday activities, physical fitness, and high quality of life. In any organism, health can be said to be a "state of balance," or analogous to homeostasis, and it also implies good prospects for continued survival. Wellness is a term sometimes used to describe the psychological state of being healthy, but is most often used in the field of alternative medicine to describe one's state of being.

The most widely accepted definition 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" (WHO, 1946). 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.

Health is maintained through the science of medicine, but can also be improved by individual effort. Physical fitness, weight loss, healthy eating, stress management training and stopping smoking and other substance abuse are examples of steps to improve one's health. Workplace programs are recognized by an increasingly large number of companies for their value in improving health and well-being of their employees, and increasing morale, loyalty and productivity at work. A company may provide a gym with exercise equipment, start smoking cessation programs, provide nutrition, weight or stress management training. Other programs may include health risk assessments and health screenings.

Alternative medicine can sometimes be used to improve health. However, with the lack of scientific proof through double blind testing, the placebo effect should be assumed to provide the health improvement in the case of successful alternative treatments until such testing can provide proof of any effects besides placebo. This is because as someone who feels well from their (possibly subconcious) belief in the therapies may lower their stress levels, resulting in beneficial effects on numerous factors, including blood pressure, gastrointestinal functioning, and immune response. The field of psychoneuroimmunology explores these links.

An increasing measure of the health of populations is height, which is strongly regulated by nutrition and health care, among other standard of living and quality of life matters. The study of human growth, its regulators and its implications is known as auxology.

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