In preceding portions of this work we have dwelt so fully upon the relations of air, water, food, heat, light, electricity, and various, other external agents, together with the influence of habits, to the human system in health and disease, that for our present purpose we have only to summarize the foregoing statements. The principal causes of disease may be classified as follows:

1. Abnormal conditions of the surroundings or of the relations of external agents to the human body. This class, of course, will include all errors and abnormal conditions pointed out under the head of hygiene of food and diet, hygiene of the air, and the relations of heat, light, and electricity to the body.

2. Injurious habits, which may be made to include some of the preceding, though the term is here used with particular reference to the abnormal use of various organs of the body, as excessive or deficient exercise of the nervous system, muscles, or of other parts of the body, deficient mastication of food, etc.

3. Accidents. This class may in one sense be considered as included under the first, but the term is here employed in the ordinary sense, it being intended to include in this class all kinds of surgical accidents and injuries.

In many cases, poisons in the blood, arising either from retained excretions or by absorption from without, are the source of disease; but cases of this sort are so manifestly a result of the operation of the causes mentioned in the preceding classes that they cannot be mentioned as a separate class.

As samples of the foregoing, we call especial attention to the fact that disease may be produced by excess or deficiency of heat, cold, food, drink, light, or electricity, a supply of all of which, in proper quantity, is essential to the maintenance of health. We may also call attention to the fact that among the most frequent causes of serious and often fatal diseases, are impurities in the air, in the shape of noxious gases, dust, miasmatic poisons, and the germs of infectious and contagious diseases, and probably also certain peculiar elements, the nature of which is not certainly known, but which give rise to epidemics, sometimes local in character, but often spreading over a whole continent. The foregoing are what are generally known as the exciting causes of disease. Any one of the causes mentioned may, under certain circumstances, be a predisposing cause of disease. We will now call attention to what may be most properly denominated.

The Predisposing Causes of Disease

As just remarked, any one of the causes or the classes of causes already mentioned may produce a condition of the system predisposing it to disease; but the causes which may, under all circumstances, act as predisposing causes, and are never other than predisposing in character, are those which arise from temperament, sex, age, idiosyncrasy, hereditary tendencies, climate, occupation, modes of life, etc. Of the various predisposing causes mentioned, probably the most powerful and inveterate in its tendency of all is heredity. The actual transmission of disease by heredity occurs only in very exceptional instances. The majority of cases of so-called inheritance of disease are simply cases in which the tendency or predisposition to disease has been inherited, the nature of which is simply a weakness or deficiency of vitality on the part of some portion of the organism.