_THEORIES OF DISEASE_
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 preeminently 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 microoerganism which, if not met by the
antagonistic agencies, finally pervades the whole system with its
progeny or its virus. The microoerganisms 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 anaemia.
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.