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 in typhoid fever, malaria and yellow fever. The comparative immunity of the natives to yellow fever in regions where this prevails seems to be due to their having acquired the disease in infancy in so mild a form that it was not recognized as such.
The infectious diseases are preëminently the diseases of the first third of life. After the age of forty man represents a select material. He has acquired immunity to many infections by having experienced them. Habits of life have become fixed and there is a general adjustment to environment. The only infectious disease which shows no abatement in its incidence is pneumonia, and the mortality in this increases with age. Between thirty-five and fifty-five man stands on a tolerably firm foundation regarding health; after this the age atrophies begin, the effects of previous damage begin to be apparent, and the tumor incidence increases.