Like everything else, stress can be known and understood at several levels. At the very physical level (which is most familiar to us) stress is a disease. This may arise either due to an environmental change (e.g. change of weather) or a de novo change in the internal milieu (e.g. viral attack). Incidentally, this change need not always be negative. It may be, for instance, a long awaited promotion, birth of a child, sudden success or victory in a strife situation. Physiologically too, it may not only be a reduction and deficiency but an excess of something essential e.g. the overactivity of a gland. Since the entire body — the milieu interior and the milieu exterior are one, there is a natural tendency in nature to harmonise and balance. So even an excess results in stress!