Disease may be defined as an abnormal condition of the human body, and since there is no one condition of the human body which can be satisfactorily described as normal, there is, therefore, no exact definition of disease.

What is disease for one person because of a departure from his normal health might not be recognized as disease in another person of different normal vitality. Nor is it possible to assign any particular and special cause for disease since the condition recognized as disease is the result, usually, not of one but of a series of causes or circumstances more or less connected and linked together, and in many cases not obviously associated with the resulting disease. Thus, in records of death, it is very common to see reported pneumonia as the cause underlying and fundamental, when the cause was really typhoid fever, the patient yielding to the former disease because of the enfeebled condition due to the latter. Again, many children contract diseases like measles or whooping cough because of reduced vitality due to insufficient nourishment, lack of clothing, and neglect, and their illness is said to be due to measles or whooping cough when under proper conditions of care and attention they would not have the disease at all. The causes of disease therefore may be divided into two classes, direct and indirect. In the latter class are to be included such causes as environment, heredity, age, and occupation. In the former class are to be found such causes as the introduction of disease germs into the system; the action of poisons, whether introduced into the alimentary canal or into the lungs, and such external conditions as excessive heat and cold and accident.

Effects of dirt.

At one time it was thought that diseases could spring up in the midst of dirt, and one of the strong arguments for keeping houses clean, for removing manure piles, and cleaning up back yards, was the fear that without such care diseases might be induced in those living near by. This is possible in a certain sense, but unless the seed or germ of the disease is present in a pile of dirt there need be no fear of the disease being developed. There is, however, a probability that by the organic decay and the consequent pollution of the atmosphere the vitality, energy, and resistance of the individual in the vicinity may be weakened.

It is well known, for instance, that prisoners confined in damp dark cells lose vitality, and when released, have but little of their former physical strength. In the chapter on Ventilation, it has been shown that persons confined in a small room and breathing their own exhaled air may in time become unconscious and die, and therefore it is reasonable to believe that persons living in the immediate vicinity of decaying animal or vegetable matter will suffer a loss of vitality and will have less resistance to disease.

Blood resistance.

It is well known that there are present in the body certain agencies which act as guardians of the body against disease; that there are certain corpuscles of the blood and certain liquids circulating through the system which immediately attack and if in sufficient numbers or strength drive out the advancing enemy, so that "taking a disease" in most cases means that the activity of these resisting organisms is not forceful enough to successfully combat the germs of the disease. These agencies, whether circulating liquids or cells or corpuscles, are most active in the healthy body, and anything that tends to reduce the general health, such as exposure, overexertion, imperfect nourishment, overeating or overdrinking, or lack of sleep, tends to diminish their activity and so makes the individual more susceptible to disease.

Cell disintegration.

Although disease is caused by the attacks of germs, another and far more important cause of disease is the breaking down or overstimulation of some particular organ. This is very plainly seen in diseases involving the stomach or intestines, where habitual excesses in eating lead, sooner or later, to consequent inflammation, disease, and death. This is also true of the lungs; merely living in an atmosphere full of dust will irritate the lungs to such a degree as to cause inflammation. Cancer is presumably the result of local inflammation, although the cause of the original suppuration is unknown. Similarly, appendicitis starts from some irritating cause, resulting in inflammation and the formation of pus. In very many cases the cell-disintegration seems to be a matter of heredity.

Heredity.

Heredity, the second of the indirect causes of disease seems to be assuming less importance as it is more studied. Probably in but few cases is heredity more than a chance factor in the causation of disease. Heredity, formerly considered to be the most important cause of consumption, is now understood to have little to do with this widespread epidemic, although it is agreed that children brought up in the family with a consumptive mother and father are more likely to contract the disease than if they were segregated.

It is a providential arrangement that children inherit the tendencies of both father and mother, and that the good qualities of one parent are known to offset the bad qualities of the other; probably for this very important physiological reason marriage between near relatives, where both parents would be inclined to the same weaknesses, has always been proscribed. However, even with the characteristics of the father offsetting peculiarities of the mother, it is possible for the traits of a parent to be reproduced in children, and this applies to mental traits as well as to physical. In some families there exist tendencies toward nervous diseases, such as epilepsy and insanity, although it is not accurate to say that either disease is naturally inherited. It has been observed that a tendency to cancer, to scrofula, and to rheumatism runs in certain families, but this is hardly more than saying that in certain families, where the predisposition in this direction by one parent is not offset by the tendencies of the other parent, the physical condition of the child is such as to encourage the development of diseases.

Age and sex.

As indirect causes of disease, age and sex cannot be overlooked. It is well known, for instance, that certain diseases belong essentially to childhood, measles and scarlet fever being markedly prevalent among children under ten years of age. In fact, it has been said by experts that if measles could be kept from children under five years old, the disease would be practically stamped out, since beyond that age they are less susceptible and the course of the disease is much milder. No greater mistake can be made than in exposing children to so-called "children's diseases" because of a desire "to have it over with." Not only is such exposure foolish, since it is quite possible to escape the disease altogether if in the first few years of life it is avoided, but also inviting death, since the mortality of the disease becomes markedly less and less as the age of the patient advances.

Many of the diseases of children are due to imperfect and incomplete development; either the lungs or the stomach or some other organ is not equal to its work, and the child remains an invalid or dies. Many children die from imperfect nutrition, especially in the second summer, when teething is at its height, on account of the ignorance of the mother and on account of unsanitary surroundings. No movement is more promising in the way of prolonging the lives of children than that recently inaugurated in New York which undertakes to teach mothers, of foreign nationality in particular, how to dress, bathe, feed, and bring up their children.

Another reason why disease occurs more frequently among children is, as will be seen later, that one attack of a disease frequently confers immunity upon the patient, so that, for example, a child having scarlet fever is not likely to have the disease later on in life; but this is no argument for exposing one's self to contagion, since it is quite possible that even the first attack may be avoided. Tuberculosis or consumption is preËminently a disease of youth, as is also typhoid fever. It is very rare for the latter disease to appear in children or in adults over forty-five, and for the former to develop until maturity.

In old age, diseases occur due to the gradual failure of the different organs to perform their normal functions. Some of these diseases are connected with the heart and the circulation, others with the liver or with the mucous membranes, so that among those advanced in life, rheumatism, gout, cancer, and diseases of the kidneys are very apt to occur.

One of the objects of sanitation is to eliminate disease due to bacteria and to prolong the normal life, so far as is possible, past the early period when diseases are easily contracted. It is not hoped that death can in any case be prevented, but hygiene will have done its utmost when death occurs only among the aged and when the diseases then causing death are only those which are consequent upon the wearing out of the body.

So far as sex is concerned, the ordinary rules of hygiene or the violation of those rules seem to have but little concern. It is generally understood that males are on the average shorter-lived, by a few months, than females, and all statistics support this position. Some diseases, like typhoid fever, attack males more than females in the ratio of three to two, while cancer attacks females to a greater extent than males at about the same ratio reversed. Generally speaking, however, excepting in so far as their occupations and manners of living make different their vital resistance, the principles of hygiene are not affected by the incident of sex.

Occupation.

Inasmuch as this discussion is a part of rural hygiene and is assumed to apply to only one occupation, namely, that of cultivating the soil, or of raising stock, it may not be considered pertinent to discuss the effect of occupation on disease. It is worth while pointing out, however, that occupation is a very important factor as an indirect cause of disease, and that one's chances of life are vastly greater in the open country surrounded by hygienic conditions than in a city in crowded quarters, confined for long hours each day at some unhealthy occupation.

As a general warning, it may be stated that a factory containing a dust-laden atmosphere is most undesirable, and this is particularly so when the dust is mineral dust. In the country, the only comparison of conditions possible is between that of the outdoor worker and that of the indoor worker; enough has already been said upon the value of fresh air and its improving effect on the vital resistance to make further repetition unnecessary. Unfortunately, in the past the occupation known under the general term of farming has not made itself conspicuous in statistics for healthfulness; but this has been undoubtedly due not to the lack of the value of the outdoor part of the farmer's life, but to the monotony of the work and to the very bad conditions found indoors, particularly in the winter. When this indoor life has been modified so that plenty of fresh air is supplied day and night, and when reasonable attention is paid to the demands of the body in the matter of food and drink, then the duration of life of farmers will rank high in comparison with other occupations.

Direct causes of disease.

The direct causes of disease may be due to the introduction into the human body of a specific microÖrganism which, if not met by the antagonistic agencies, finally pervades the whole system with its progeny or its virus. The microÖrganisms thus responsible for disease are commonly divided into two classes, namely, parasites and bacteria. In the first group are included those parasites that cause tapeworm, malaria, trichinosis, and hookworm; in the second group those bacteria that cause typhoid fever, cholera, erysipelas, diphtheria, and probably smallpox, measles, scarlet fever, chicken pox, and a number of others presumably similar.

Parasites as causes of disease.

The introduction of worms into the body must come either from impure drinking water, from impure food, or from the bites or stings of insects. When introduced into the body, those parasites that are inimical to man and produce abnormal conditions interfering with usual physiological functions may or may not develop further. In some cases, as in malaria, the very act of hatching the malarial brood is sufficient to throw the host on whom the brood will feed into a violent chill.

In other cases, as with the hookworm, while eggs are produced in the human body, they have no directly detrimental effect, the objectionable feature of their residence being due to the fact that the continual draught which they make upon the blood vessels of the intestine reduces the vitality, causing anÆmia.

In other cases, as with the guinea worm, found in Africa and South America, the worm wanders from the stomach, which it enters toward the surface of the body, and finally breaks through, causing ulcers or abscesses.

In still other cases, as with that form of filaria which causes elephantiasis, the adult worm or the embryos are present in the lymphatics in such numbers as to interfere with circulation, causing the fearful swellings characteristic of the disease named.

Finally, in such cases as trichinosis and tapeworm, there is usually but little inconvenience to the human being harboring them, except when their number becomes very large. Then there may be diarrhoea, loss of appetite, and other digestive disturbances. The different tapeworms are generally responsible for nothing more than indigestion and nervousness. These latter parasites are, however, formidable in so far as their size is concerned. The mature pork tapeworm is about ten feet long, although the eggs, seen in the pork flesh, giving it its name of "measly," are only about a thousandth of an inch in diameter. The fish tapeworm, when mature, measures about twenty-five feet in length, while the beef tapeworm is about the same length. These worms can develop only in the bodies of the animals named, and find their way into the human body only through the medium of imperfectly cooked meat.

If proper precautions be taken in these directions, if only water is used for drinking which is known to be free from such parasites and their eggs, and if insects like mosquitoes and fleas are kept away by screening windows and doors, and if meat be always thoroughly cooked, the dangers of diseases from parasites will be reduced to a minimum.

Bacterial agencies.

By far the most important of the living agencies concerned with the direct production of disease are those small vegetable organisms known as bacteria. Not all bacteria, by any means, produce disease; in fact, it is not too much to say that the majority of bacteria are benefactors to the human race. Their chief agency is not to cause disease, but to prevent it, and they do this because they are able to transform the waste products of animal life, which would normally be dangerous to health, into harmless mineral residue. They are really the scavengers of the earth's surface, not actually carrying off garbage, but rather transforming it, and, in the process, not merely destroying it, but changing it so as to make it available for plant-food. It is through the agency of bacteria that the air, which is being continually overloaded with carbonic acid from the lungs of animals, is reduced and taken up by plants so that an equilibrium is maintained. Otherwise, the atmosphere would be more and more vitiated with carbonic acid and organic vapors, and every one would die as if shut up in an air-tight room. But, because of bacteria, neither is the surface of the earth overloaded with waste organic matter nor do streams, however much polluted, continue to flow without some improvement being traced in their quality.

In some of the ordinary manufacturing processes, bacteria are all-important, as in making vinegar, wines, cheese; in fact, in any of the fermented food products. In agriculture, they are entirely responsible for supplying an adequate amount of food material to growing plants. Fresh manure is not suitable for plant-food and would be of no value on the fields or in the garden except as improved and modified by bacterial action. One of the greatest discoveries of their importance recently made has to do with the way in which peas and beans are able to absorb nitrogen from the air through the agency of bacteria. One knows that plowing under a crop of peas or clover enriches the soil, and that peas or clover make the best growth for this purpose. The reason is that these plants, through the activity of bacteria, are able to absorb nitrogen from the air and afterwards to convert it into food material.

But with all these good qualities a few bacteria, gone bad, perhaps, are associated with diseases, and by a series of experiments, chiefly those of a Frenchman named Pasteur and of a German named Koch, and of their followers, it has been ascertained that certain bacteria, and those only, will cause certain diseases. These diseases, that is, these caused by bacteria, are generally spoken of as epidemic or contagious, of which typhoid fever and cholera are examples.

All contagious diseases cannot at present be definitely associated with bacteria, probably for the reason that the methods employed to find the bacteria have not been adequate. For instance, the bacteria of smallpox has never been found, although the disease is so characteristically one of bacterial origin that no one can doubt the cause. Similarly, the bacteria responsible for measles, scarletina, and whooping cough have never been discovered, although the cause of each is also presumably bacterial. More definite information on the subject of the individual and responsible bacteria will be given in the subsequent chapters dealing with specific diseases. Inquiries into the method of growth and into the life history of specific bacteria serve our present purpose only as they teach methods for the prevention of the disease. For example; when it was found that the parasite of yellow fever, in the course of its life, spent fourteen days in the mosquito's body in such a condition that the mosquito during that time was harmless, it made possible exposure to mosquitoes laden with yellow fever for a period of thirteen days from the time of the preceding case.

Antitoxins.

But the methods of combating the different diseases when once contracted in the human body, based on the knowledge obtained of the life history of these germs, have been the most important result of their biological study. A large part of this knowledge has been acquired by the study of animals which have been found susceptible and so available for experimental investigation, and it may be that the impossibility of studying measles, for instance, in animals, may be one reason why the germ has never been discovered.

There is no evidence that animals suffer spontaneously from such diseases as typhoid fever, Asiatic cholera, leprosy, yellow fever, smallpox, measles, and so on; but it seems that in animals, as in man, the disease is the direct result of the life and growth in the animal of the characteristic disease-producing germ. The fact that diphtheria or tuberculosis can be experimentally given to rabbits or guinea pigs is without doubt the chief source of our knowledge of those diseases, although, in general, it is impossible to produce diseases in any animal which will be, clinically, precisely like the disease as it appears in man. The converse of this is also true, namely, that when it has been found impossible to experimentally inoculate an animal with a disease supposed to be bacterial in nature, then but very little of that disease is known.

The most important result of bacterial studies has been the production of what are known as antitoxins, and no more wonderful discovery has ever been made. To understand as best we may the principle involved, it is necessary to explain the process of bacterial attack. When bacteria capable of producing disease are introduced into the system, either through the mouth or into the lungs or into the blood through some skin abrasion, the bacteria, finding there a congenial habitat, thrive, grow, and multiply. In some cases, this bacterial growth results only in breaking down the cell tissues at the point or in the vicinity of the place where growth occurs; for instance, if a cut is made with a dirty knife, that is, one carrying bacteria on the blade, and is not immediately washed out with an antiseptic solution, bacteria will grow and pus will form in the cut. Similarly, a splinter, if not removed and cleansed, will produce a pus-forming wound. But unless a very extensive suppuration starts, the difficulty is all local. So it is with consumption, when the bacteria are localized in the lungs and by their growth destroy the lung tissue without, at least for many weeks, affecting the general health.

There are germs, however, like typhoid fever and diphtheria, which do not produce any particular local disturbance with the growth of bacteria, but the whole body becomes sick, the circulation of the blood is affected, and a general disturbance ensues. This is due to the action of a poison, called a toxin, which is set free as a result of the growth of the bacteria in some one part of the body, which poison is then carried by the blood throughout the entire system, inducing fever and a general debility.

Just how these toxins are formed is not certain. They are not the bacteria themselves. This we know because the disease-producing bacteria can be grown in broth and the mixture can be strained through fine porcelain, fine enough to strain out the bacteria. Yet it has been found that the clear liquid passing the porcelain filter is capable of producing disease and is a deadly poison without the presence of any bacteria at all. During the incubation period of a disease, as, for example, in the three-week period when typhoid fever is developing, these poisons are being formed and are being scattered through the body, and it is during this time that the fight takes place between these poisonous forces and the defending forces always present in the human system. As already pointed out, these defensive forces are powerful or not, according as the general health of the individual is good or bad, and we see the familiar sight of persons said to be run down taking a disease, while those not so depleted of vitality are able to resist or remain immune.

So certain are scientific men of this power and of the fact that the power resides generally in the white corpuscles of the blood that, in the presence of a dangerous infection, a person's blood may be examined, and, if the white corpuscles are not present in sufficient quantity, proper means must be taken for developing this element in the blood, or else the person must take himself away from the infection, if the infection is to be avoided.

As a result of the conflict between the toxins and the defensive forces of the body, certain vital processes are set free in the blood and in the cells which seem to possess a highly specialized power of defense against any subsequent attack. Pasteur, in his researches on the subject of rabies, developed this power of resistance by inoculating into rabbits the rabies infection of a monkey. Monkey rabies is not a severe form and is scarcely felt by the ordinary rabbit, but if the infective material (usually part of the spinal cord) of the monkey-infected rabbit is transferred to a second rabbit, the disease becomes more severe; and if the disease is passed from animal to animal, it may be built up into as severe a form as desired, up to the maximum. Pasteur found that by inoculating an individual with a one-day rabbit, that is, with the weakest brand of infection killing a rabbit in one day, and the next day with a two-day rabbit, that the person could receive this two-day inoculation without discomfort or danger because of the greater antagonism acquired by the preceding inoculation. Continuing the inoculations for fourteen days and making the strength of the infection stronger each day, at the end of the period it was found that the fourteenth inoculation, strong enough to produce the disease and kill a fresh subject, had, on account of the preceding inoculations, produced ability to withstand or counteract the actual disease developing perhaps at the same time. Fortunately, in the case of this disease, the shortest period for its development is fifteen days, and often it is a month or more after the bite of the dog before the disease develops. By successive inoculation of increasing strength for fourteen days, the system will have acquired a habitude to the disease which prevents the normal effects.

Diphtheria is prevented in much the same way, except that in this case horses are used, their blood being strengthened to resist the disease by successive inoculations of the diphtheria poison. It is probable that all the bacterial diseases which exert their influence through the transmission of toxins in the blood may be counteracted by the production of an antitoxin when once the method of building up this antitoxin has been learned. At present, rabies, tetanus, diphtheria, and cerebrospinal meningitis are the four diseases for which antitoxin is made commercially and generally used. For a great many years, scientists have labored without success to find an antitoxin for consumption, and within the last year extensive experiments have been made in the American army on the use of antitoxin for typhoid fever.

Natural immunity.

It may be worth noting that not all resistance to specific diseases needs to be acquired in the roundabout way just described. The state of being free from disease is known as immunity, and the way of securing immunity just described is known as artificial immunity. This artificial immunity may also be obtained in the course of events by having the disease as a child, thereby generating the antitoxin in one's own body instead of in the body of some cow or horse or rabbit.

There is, however, a natural immunity which is due to long-continued environment or to protracted heredity. The negroes in the South have, by a lifelong proximity and struggle with the disease, acquired a practical freedom from typhoid fever, although it remains with the negro sufficiently to form a focus for the spread of the disease among others not equally immune. Creoles in yellow-fever districts have a natural immunity from the hookworm disease, although probably the class are responsible for its generous transmission to the poor whites with whom they associate. Racial immunity from certain diseases may be shown by statistical studies.

Chemical poisons.

Instead of the introduction of toxins into the body by the agency of bacteria, it is quite possible for chemical poisons, not formed originally by bacteria, to be set free in the body. Sulphate of copper, for instance, is essentially a mineral poison which acts on the human system in such a way as to produce death, and certain other mineral substances may be mentioned, such as phosphorus, arsenic, and mercury, which are well-known poisons. There are also many vegetable products, not bacterial, which are poisonous in their nature, that is, distributing to the blood and lymphatics certain substances in solution which act on the cells of the various organs of the body in such a way that the activity of those organs is stopped. Opium, cocaine, alcohol, and some of the coal-tar products used for headaches, as phenacetin, are deadly poisons when a limited dose is exceeded.

There are also certain poisons engendered in the body itself whose action is similar to that of chemical bodies and which can hardly be called bacterial. These poisons represent generally stages in the process of nutrition where for some reason the normal process is arrested and chemical bi-products are set free. Also, tissue which has been thrown off, in or by any organ, begins to decompose, thereby sending throughout the system the poisons of decomposition. Inflammation too generally results in the breaking down of the cells and the distribution of the resulting poisons. Of late years, much has been said of the poisonous property of the body waste not disposed of by excretion, and the theory of auto-intoxication, so-called, has received many adherents. The great scientist, Metchnikoff, has even gravely contended that it would be well for children to have their larger intestine removed entirely, because in that organ putrefaction occurs, the cause of the auto-intoxication he would try to prevent.

External causes.

The external causes responsible for disease are due to conditions of weather so severe as to be outside the possibility of self-protection. Excessive heat is responsible each year for deaths from sunstroke, and other conditions of weather are often the direct causes of disease, if not of death.

Accidents are the indirect cause of death, and there will always be a small proportion of the deaths occurring each year due to violence or accident. But, inasmuch as these deaths are clearly preventable, it is the duty of those interested in rural hygiene to study the reasons for accidental death, and, if the number of such accidents can be reduced, to strive for that reduction. As an example, it may be mentioned that each year a number of deaths in New York State, and probably in other states, occur from accidents at culverts and bridges, due to insufficient protection in the way of railings and fences. A method of reducing the deaths from accidents, therefore, would include a proper survey of all the roads of a vicinity to make sure that no danger exists in this regard. Other precautions against preventable accidents will readily suggest themselves.