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.