ABSTRACT
Stress is a word used with various meanings. Many of the components of the stress concept are embraced by saying that stress is a change in homeostatic systems which, if persistent, threatens an individual’s fitness. Unfortunately, such a definition introduces a further term of some complexity: ‘fitness’; but for many purposes this can be equated with health—if that helps! There is also an element of tautology in the definition, since homeostasis itself and health can amount to the same thing. However, there are many settings of homeostatic mechanisms which are compatible with good health or high fitness. The important positive components of the definition are that it refers to a state of the person, not to a state of the environment (for an environmental factor which produces stress in one individual may not in another); and that it is always a harmful or potentially harmful condition, never beneficial. One can thus, on this definition, never have ‘too little’ stress. If physical exercise is typically beneficial then the physiological responses to it are not stresses. On the other hand, the definition does permit consideration of diseases which are not normally regarded as stress diseases—as for example the effects of infectious diseases, though of course these can be excluded by caveat.
