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 spirochaeta 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.