Biological science does not try to distinguish between health and disease. Biology is concerned with the interaction between living organisms and their environment. What we call health or disease is quite irrelevant.

These reactions between the individual and his environment are complex. The individual and his surroundings form an integrated system which we can arbitrarily divide into two parts. There is an “external” component, by which we mean such factors as light, heat, percentage of oxygen in the air, quantity of minerals or vitamins in food, micro-organisms in food or air, and so on. These can induce changes in what we arbitrarily call the “internal” component. Here we include such crude factors as anatomical structures, or finer details like composition of intercellular fluid, or secretions of glands, or changes of electrical potential in nerve or muscle.