It has been by imitating nature’s methods and by augmentation of the natural forces of defense, that good has been achieved… Common observations early indicated that individuals of all animal species, and of the human species especially, were very unequally subject to disease. This elementary fact is impressed everyday upon the thoughtful and has been, from the earliest times, the object of much ingenious speculation. By considering the various theories which have been offered in explanation of acquired immunity, this book deals with the fact of natural resistance to infectious disease.