There is growing evidence that stress is a major factor in maintaining health and well-being.  Health means more than the absence of illness.  In China and in ancient Greece health was thought of as being in balance with nature.  The vital task was seen as maintaining equilibrium in the face of extensive demands.  The World Health Organization (WHO) definition of health is a state of complete physical, mental and social well being.

It seems increasingly clear that the naive notion that a germ automatically leads to illness is no longer acceptable, although much of the lay public subscribes to the theory. Today we know that the idea of a single external cause, such as a germ, is oversimplified. The presence of the germ does not always cause the illness. The vulnerability of the person or the host animal is a major factor and that vulnerability is influenced by such factors as immunity and stress levels. So the cause of illness is not quite as simple as we once thought.