Disease and Its Causes by William Thomas Councilman (early reader books .TXT) 📕

is known as cowpox, in which vesicles similar to those of smallpox appear on the skin. The inoculation of man with the contents of such a vesicle produces a mild form of disease known as vaccinia, which protects the individual from smallpox. This protection is fully as adequate as that produced by an attack of smallpox, and we are warranted in saying that if thorough vaccination, or the inoculation with vaccinia, were carried out smallpox would disappear. There are great difficulties in the way of carrying out effective vaccination of the whole population, which are accentuated by the active opposition of people who are ignorant and wilfully remain so. There exists in every state a number of people unprotected by vaccination, and among these single cases of smallpox appear. The unprotected individuals gradually increase in number, forming an inflammable material awaiting the spark or infection which produces a conflagration in the one case and an epidemic in the other.

Cerebro-spinal meningitis is another example of a disease which exists in sporadic and epidemic form. This disease is caused by a small micrococcus, the organisms joined in pairs. The seat of the disease is in the meninges or membranes around the brain and spinal cord. The micrococci enter the body from the throat and nose, and either pass directly from here into the meninges, or they enter into the blood and are carried by this into the meninges. The organisms are easily destroyed and cannot long survive the conditions outside the body, so that for infection to take place the transmission must be very direct. Carriers who have the organisms in the throat, but who do not have the disease, are the principal agents in dissemination. The mortality is high, and even in recovery permanent damage is often done to the brain or to the organs of special sense. Sporadic cases constantly occur in small numbers, and it is difficult or impossible to trace any connection between these cases. At varying intervals, often twenty years intervening, an epidemic appears which sometimes remains local in a city or state, sometimes extends to adjoining cities or states, and may even extend over a very large area. In the epidemics the mortality is much higher than in the sporadic cases. The same explanation given for smallpox cannot apply here, for there is not a similar accumulation of susceptible material. We know there is a great deal of variation in the virulence of the different pathogenic organisms, and the virulence can be artificially increased and diminished. In epidemics of meningitis the virulence of the organisms is increased, as is shown by the greater mortality. It is highly probable that such epidemics are due to changes which arise in the organisms from causes we do not know and which increase their capacity for harm. It is possible that such a change would convert a carrier into a case of disease, the organism acquiring greater powers of invasion. Such a strain of organisms arising in one place and producing an epidemic could be transported to another locality and exert the same action, or similar changes in the organisms could arise simultaneously in a number of places. Analogies to such conditions are given in plants. In certain plants it has been shown that from unknown causes there appears a tendency to the production of variations. A very beautiful herbaceous peony known as "Bridesmaid" after having grown for a number of years in single form, in one year wherever grown suddenly became double. The peculiar thing with the lower unicellular organisms is that the changes which so arise do not tend to become permanent, the organism reverts to its usual character, the disease to its sporadic type.

A very fatal form of poliomyelitis has for a number of years prevailed in Sweden. In the United States there have been continually a number of single cases of the disease, and it is not impossible that a more pathogenic strain of the organism has developed in Sweden and has been imported into this country, giving rise to the much greater extension of the disease in a number of places.

The most cursory study of the infectious diseases shows that there is great variation in the susceptibility of individuals. Even in the most severe epidemics all are not equally affected, some escape the infection, others have the disease lightly, others severely, some die. Chance enters into this, but plays a small part, for the same varying individual susceptibility is shown experimentally. If a given number of animals of the same species, age and weight, even those from the same litter, be inoculated with a given number of bacteria shown to be pathogenic for that species, the results differ. If the dose be necessarily fatal, death will take place at intervals; if a dose smaller than the fatal be used, some animals will die, others will recover. The defences of the organism being centred in the activity of the living tissue, any condition which depresses cell activity may have an effect in increasing susceptibility to infection. Animals which ordinarily are not susceptible to infection with a certain organism may be made so by prolonged hunger, or fatigue, by the influence of narcotics, by reduction of the body temperature, by loss of blood. In man prolonged fatigue, cold, the use of alcohol to excess and even psychic depression increases susceptibility. It has been shown that such conditions are accompanied by a diminution in the power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different races of man. If a race be confined to one habitat with close intercourse between the people, such a race may acquire a high degree of immunity to local diseases by a gradual weeding out of the individuals who are most susceptible. A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals. These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host. Diseases in animals greatly increase under the artificial conditions of domestication. Certain highly specialized breeds of cattle, as the Alderneys, are much more susceptible to tuberculosis than the less specialized. The high development of the variation which consists in a marked ability to produce milk fat is probably combined with other qualities, shown in diminished resistance to disease, and under natural conditions the variation would not have persisted. The introduction of a new disease into an isolated people has often been attended with dire consequences. It is much the same thing with the introduction of disease of plants. In Europe the brown-tail moth and the gypsy moth produce continuously a certain amount of damage to the trees, but their parasitic enemies have developed with them and check their increase. These pests were brought to this country in which there were no conditions retarding their increase and have produced great damage.

It is very difficult to estimate the degree of racial susceptibility. The negro race seems to be more susceptible to certain diseases, such as tuberculosis and smallpox, less so to others, as yellow fever, malaria and uncinariasis. What are apparently differences in susceptibility may be explained by racial customs. A statistical inquiry into death in India from poisonous snakes might be interpreted as showing a marked resistance on the part of the white to the action of the venom, but it is merely a question of the boots of the whites and the naked feet and legs of the natives. The relatively greater frequency of smallpox in the blacks is due to the greater difficulties in carrying out vaccination measures among them and the greater opportunity for infection which results from their less hygienic life. It has always been noted that when plague prevails in Oriental cities, the natives are more frequently attacked than are Europeans. This does not depend upon differences in susceptibility, but on the better hygienic conditions of the whites which prevent the close relation to rats and vermin by which infection is extended. There would be but little extension of the hookworm disease in a community where shoes were worn and the habits were cleanly.

It is by no means improbable that the formation of the habits of civilization was influenced by infection. Most of these habits, such as personal cleanliness, the avoidance of close contact, the demand for individual utensils for eating and drinking, are all of distinct advantage in opposing infection. Certain habits, on the other hand, such as kissing, which probably represents the extension of a habit of sexual origin, are disadvantageous and infection is often transmitted in this way. In syphilitic infection the mouth forms one of the most common localizations of the disease and may contain the causal organisms in great numbers. This, the spirochæta pallida, is an organism of great virulence, and man is the most susceptible animal. The disease, like gonorrhoea, is essentially a sexual disease, the primary location is in the sexual organs, and it is transmitted chiefly by sexual contact. Of all the infectious diseases, it is the one most frequently transmitted to the unborn child; in certain cases the disease is transmitted, in others the developing foetus may be so injured by the toxic products of the disease that various imperfections of development result, as is shown in deformities, or in conditions which render the entire organism or individual organs, particularly the nervous system, more susceptible to injury. Following the primary localization of the acquired form of the disease, there is usually secondary localization in the mucous membrane of the mouth, and the disease may be transmitted by kissing or by the use of contaminated utensils. The habit of indiscriminate kissing is one which might with great benefit be given up.

There is definite relation between age and the infectious diseases. In general, susceptibility is increased in the young; young animals can be successfully inoculated with diseases to which the adults of the species are immune, and certain human diseases, such as scarlet fever, measles and whooping cough, seem to be the prerogatives of the child. It must be remembered, however, that one attack of these diseases confers a strong and lasting immunity and children represent a raw material unprotected by previous disease. Where measles has been introduced into an island population for the first time, all ages seem equally susceptible. All ages are equally susceptible to smallpox, and yet in the general prevalence of the disease in the prevaccination period it was almost confined to children, the adults being protected by a previous attack. The habits and environment at different ages have an influence on the opportunities for infection. There is comparatively little opportunity for infection during the first year, in which period the infant is nursed and has a narrow environment within which infection is easily controlled. With increasing years the opportunities for infection increase. When the child begins to move and crawl on hands and knees the hands become contaminated, and the habit of putting objects handled into the mouth makes infection by this route possible. Food also becomes more varied, milk forms an important part of the diet, and we are now appreciating the possibilities of raw milk in conveying infection. With the enlarging environment, with the school age bringing greater contact of the child with others, there come greater opportunities for infection which are partly offset by the increase in cleanliness. The dangers of infection in the school period are now greatly lessened by medical inspection and care of the school children. In the small epidemic of smallpox which prevailed in Boston from 1881 to 1883, there was a sharp decline in the incidence of the disease in children as soon as the school age was reached, this being due to the demand of vaccination as a condition for entrance into the schools. Many of the infectious diseases are much milder in children than in adults. This is the case