“It is well known that certain individuals are more severely affected by any ascertained cause of disease than others; and also that the same exciting cause may at one time produce serious disturbance of health, while at another, and under precisely the same conditions, as far as known, no injurious effect is produced. How frequently do we observe during the same epidemic, as, for instance, scarlet fever, measles, diphtheria (and even of sporadic forms of disease), a marked difference in the character and severity of individual cases. Even in members of the same family, under apparently similar conditions, some are stricken down with the most malignant form of one of these diseases, while others may, at the same time, be but slightly affected by it, or perhaps entirely escape an attack. It can not be that they who are the most severely affected receive a larger or a stronger dose of the morbific agent which has produced the disorder, than the others, and that the disease-producing influence, in consequence of larger quantity or greater strength and power, acts with more severity and force on one than on another. For, leaving out of consideration all effects of existing predisposition, we know that a person unprotected by a previous attack or by vaccination, would be, in all probability, just as severely affected by the contagious influence of a case of small-pox, whether he was exposed for a few moments or for several hours; and besides, it would make no difference whether the case happened to be a mild one or of a more malignant form.
“It is, therefore, difficult to account for this variable operation of disease-producing agents, unless we admit the existence of such a latent predisposition as that already mentioned, and acknowledge that the system, at the time of exposure to disease-producing causes, is thereby made more or less susceptible to their effects in proportion to the development of such a predisposition. The less the power of resistance and the greater the degree of impressibility, the more aggravated will be the character of every disease which affect the system while it is thus predisposed; or, in other words, the severity of the disease will be proportionate to the degree of departure from the standard of health.”[2]