Within less than one year after the adoption of approved methods for the purification of the water-supply of Lawrence, Mass., the death-rate from typhoid fever was reduced nearly 59 per cent., and in Chicago the deaths from this disease were diminished approximately 60 per cent. within a year after the domestic water-supply was obtained from a non-polluted

source.

As a result of the proper drainage of soils, a diminution in the frequency of pulmonary, intestinal, and malarial troubles has everywhere been observed.

These few illustrations, not to mention the advantages that have accrued from increased attention to personal hygiene, to diet and raiment, to the laws of disinfection, isolation, and quarantine, should serve as convincing proof that the efforts of the hygienist have not been in vain; that they have not only been of enormous benefit to mankind, but that with the increased store of knowledge that is constantly accumulating, they are still further capable of such benefits. Already countless lives have been saved; we are told that the longevity of the human race has been increased, and in every way the conditions under which man lives are better than they were a few years back.

In citing, as illustrations, the advances that have been made along the lines of hygiene and the good that has been derived from them, it is not our desire to leave the impression that the millennium has arrived; that our stock of knowledge on the subject is complete or satisfactory in all details; or indeed even that the knowledge we possess is utilized to the extent that its importance demands. When we realize that the majority of all deaths is still from preventible causes, most of which are already quite familiar to us, it is manifest that this must be in a large measure due to an indifference on our part to put into practice even that knowledge which we already possess for their prevention. The great majority of deaths result from infection, from insufficient attention to diet, and from want of care with regard to the temperature of the body-that is to say, they are the direct outcome of our surrounding circumstances. We consider it quite within the

bounds of moderation and discretion to declare that, by the indefatigable practice of the sanitary precepts now known to be sound with regard to the prophylaxis and management of the commoner infectious diseases, to the hygiene of infancy, to diet and clothing, the death-rate from preventible causes could be conspicuously reduced, and this, too, without the addition of a single new fact to the knowledge that we already

possess.

SECTION I.

THE CAUSATION OF DISEASE.

As has been stated, the object of hygiene is to prevent disease. It is therefore necessary to an understanding of the means employed in securing this end that we possess a clear comprehension of the factors concerned in the causation and dissemination of disease.

The causative factors in disease are manifold; they differ in nature the one from the other, and are of varying degrees of importance in their relation to morbid conditions.

In considering this phase of our studies it is well to bear in mind from the beginning that beyond traumatisms and direct poisons there is probably no single, absolute cause of disease, but that the abnormal state we call disease represents the termination of a chain of circumstances the various links of which, while having more or less of a direct bearing upon the others, are of different degrees of importance to the process. Thus, by way of illustration, we say that tuberculosis is caused by a specific micro-organism, and no one doubts this; but at the same time there is no one who believes for an instant that, if to a number of individuals in sound health this micro-organism gains access, tuberculosis will with certainty. result in all cases. There are other factors that come into play and must be taken into consideration. On the one hand, there are circumstances that modify the disease-producing powers of the micro-organism, so that at one time it may be comparatively feeble as regards this property, while at another it is infective to the fullest extent. On the other hand, there are modifying influences constantly at work upon the individual, some of them placing him in a condition to survive to the most virulent forms of infection, while others

exposure

so modify the normal vital resistance with which nature has provided him that he readily falls a prey to what would otherwise be a comparatively insignificant foe. In other words, certain influences to which man is exposed during the course of his existence predispose him to disease in general, while others are concerned in directly exciting certain definite groups of symptoms and pathological manifestations, which are usually classed as specific diseases. In considering the causation of definite or specific diseases it is impossible to ignore those surrounding conditions that predispose to them. Reverting to our illustration, the exciting or direct cause of tuberculosis, with all its varied expressions, is Bacillus tuberculosis, while the predisposing or indirect causes may be numerous—as age, race, occupation, unsanitary surroundings, and heredity.

In their relation to specific disease-processes in general, the causative factors are therefore usually classed as Exciting or Direct and Predisposing or Indirect.

Predisposing Causes of Disease.-By this term is usually meant those conditions with which man is surrounded that have a tendency so to reduce his normal vital powers that he is no longer capable of resisting the inroads of the direct, exciting causes of disease. In addition to the artificial influences that surround man, there should be included in this category such natural factors as age, sex, heredity, race, etc., that in some cases exhibit a temporary, in others a continuous, influence in this respect.

The term "vital" or "animal resistance" was at one time vaguely employed in explanation of the efforts of the tissues to evade infection. In the light shed by modern investigation upon the means of defence possessed by the animal organism for the resistance of disease, vital resistance signifies a group of animal functions, some of which are capable of ready demonstration, that is possessed by every living being in health, and through the exercise of which the body. is enabled to withstand, within limits, the influence of detrimental agencies.

We now know that the circulating fluids, certain wandering cells, and certain fixed cells of the animal economy are

directly antagonistic to many of those particulate causes of disease known as micro-organisms; that through the exercise of these vital functions invading bacteria are often destroyed before sufficient time has elapsed for them to multiply and evolve their products which are instrumental in disturbance of function and destruction of tissues. We have reason for believing, moreover, that within these same fluids and cells lies also the power of neutralizing to a certain extent the poisonous products of bacteria which themselves may not be destroyed by the body.

The highest expression of this vital phenomenon is naturally found in the healthy being. Agencies that tend to reduce the general health, such as exposure, fatigue, malnutrition, debauch, etc., tend likewise to diminish the vital resistance, and in this manner render the individual more susceptible to disease.

Age. As a factor in predisposing to disease, the influence of age cannot be ignored. To certain maladies the young are more prone than the old, while to others the reverse is the case. The greatest number of deaths, and hence the greatest amount of sickness, occurs among the very young and the very old-i. e., before the age of five and after the age of sixty-five to seventy years, indicating that at these periods the vital processes are at the one time but imperfectly developed, while at the other they are declining in efficiency. There are certain diseases common to all ages, though their pathological manifestations may vary according to the age of the subject affected. For example, tuberculosis is seen in the infant, the adult, and the aged; but in childhood its expressions are most frequent in the lymphatic, the osseous, and the serous tissues, while in maturity and old age the lungs are usually involved when tuberculosis is anywhere present in the body (Louis's law).

In general, it may be said that during infancy and childhood the diseases most frequently encountered are those connected with the development of anatomical structures and the establishment of physiological functions. To this may be added those dependent upon congenital defects and upon