10. Lastly, Epidemics resemble each other in being produced by the same causes. The whole tenor of experience shows that whatever produces an especial liability to one epidemic, produces a similar liability to every other. The Causes of epidemics, as of all other diseases, are divided into two classes,—the predisposing and the primary. The predisposing causes are those circumstances which bring the body into a fit state for the action of the primary. The primary cause is the agent which directly and immediately excites the disease. If a number of persons, in an ordinary state of health, say a hundred, are exposed to the primary cause of any epidemic—to the poison of Cholera for example—probably not more than ten would be seized with the disease. Why do the ninety escape? The poison, by the supposition, encompasses and acts upon all alike: why do ten only suffer? Suppose these same hundred persons took a large dose of arsenic, or an over-dose of chloroform, not only would not one in ten escape, but every individual would certainly perish. It is conceived that the primary cause cannot take effect unless the system be in a state of susceptibility to its action; that there is in the body an innate power of resistance to all noxious agents of this kind, rendering it, when in full vigour, invulnerable to them; that there are certain circumstances which weaken or destroy this resisting power, and which even impart to the body a peculiar susceptibility to the influence of such agents—and these circumstances are called predisposing causes. The predisposing causes of epidemics may be divided into two classes—External and Internal. The external are those which vitiate the atmosphere; the internal are those which more immediately vitiate the blood. The vitiators of the atmosphere include overcrowding, filth, putrescent animal and vegetable matters of all kinds, exhalations from foul cesspools, sewers, rivers, canals, ditches, marshes, swamps, &c. Causes of this class are also called _localizing_, because they favour the generation and spread of epidemics in the localities in which they abound. The causes which more immediately act from within are those which either directly introduce pernicious matters into the interior of the body, in the shape of foul water or putrescent food; or which indirectly accumulate noxious matters within the system, by impairing the action of the excretory or depurating organs whose office it is to maintain the blood in a state of purity, by removing out of the system substances which having served their purpose have become useless and pernicious. The earnest attention which has been recently directed to the first class of causes has led to an advancement in the science of prevention, the importance of which it is impossible to over-estimate. To give only one illustration of the action of a predisposing cause, I select as my example, _Overcrowding_.