The predisposing causes of disease are generally subdivided under several headings, which may begin with the hereditary constitution, which is the most prominent and important, then temperament, age, sex, etc. - these are all intrinsic, existing within the body independently of any influence from without; then follow a number of extrinsic predisposing causes, such as excitement, occupation, and conditions which induce debility.
Excitement may be looked upon as a predisposing cause of disease when it leads to excessive development of activity in the circulatory and nervous systems associated with general or systemic excitement - a condition which may stop short of actual disease, but is especially favourable to the action of any exciting causes which tend to set up inflammation. In a state of general excitement the system is liable to suffer from any febrile disease which may be at the time prevalent, or from sudden exposure to violent changes of temperature, which may lead to an inflammatory attack. Certain forms of local excitement lead to the determination of blood to a particular part, which may end in the rupture of vessels, or may increase the functional activity of a part and thus render it liable to any influence from without or within which may induce congestion or inflammation.
Excitement may in itself result in the development of active inflammatory disorders without the intervention of any extrinsic cause, in which case it would be classed among the exciting causes of disease. For the present purpose it is only to be looked upon as acting to a sufficient degree to render the system susceptible to disease without actually inducing it. It is in the nature of things, in fact, that predisposing and exciting causes very frequently approach each other so closely that it is impossible at all times to distinguish the one from the other.
Debilitating influences, whether arising out of insufficiency of food or feeding to excess, will both have the effect of reducing the vital energy, and in this way diminishing the power to resist disease. Insufficient food is the most common cause of debility acting in a perfectly intelligible way, not only by a failure to supply a sufficient amount of nutriment to compensate for the waste of the tissues, but further by inducing a feeble condition of the digestive powers as a mere consequence of inactivity. Excess of food induces a similar result through the medium of the opposite conditions, the nutritive functions become impaired as the result of congestion of the vessels which supply the digestive organs with blood, and consequent overwork and derangement of the secretive and excretive processes. Further, mischief is done by an excess of nutritive material beyond the amount which the system is capable of appropriating, the excess being converted by oxidation into certain animal alkaloids and extractive matters, which exercise a deleterious influence on the organs and functions of the body. Next in importance to the influence of excess or deficiency of food, bad quality may be considered, the immediate effect of which will depend upon the particular constituents which occasion the deterioration. Products of fermentation, growth of fungi (moulds), decomposition, which implies the presence of septic microbes, may render food of bad quality actively poisonous or positively pathogenic, and in such cases it would come under the head of an exciting cause of disease; but short of being actively poisonous or disease-producing, the changes induced in it may merely have the effect of weakening the system without actually producing obvious disease.
All that has been said in reference to the effects of food may be applied to water, which, indeed, may be taken as representing a portion of the food.
Impure air exerts a remarkable influence upon the health of the body in two directions: (l) by failing to supply the proper amount of oxygen for the purpose of respiration and the purification of the circulating fluid; and (2) by introducing into the system organic and inorganic substances which may gradually assist in disturbing the nutritive functions and lowering the vitality of the body as a whole.