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.