trated virulence of the isolated parasite. This bacillus likewise produces spores of a persistent nature, which every consumptive patient spits broadcast into the world.
Relapsing fever is another disease of definitely proved origin. If we mention, furthermore, abscesses, the dependence of which on bacteria has lately been established, we have about exhausted the list of human afflictions about the cause of which there is no longer any doubt. Some diseases peculiar to lower animals belong also to this category. The classical researches of Pasteur have assigned the silkworm disease and chicken-cholera to the same rank. Several forms of septicemia and pyæmia have also been studied satisfactorily in animals. Indeed, the analogy between these and the kindred forms of blood-poisoning in man is so close that there can be no reasonable doubt as to the similarity of cause. This assumption, next door to certainty, applies equally to the fevers of childbirth. The experimental demonstration of the parasitic nature of leprosy, erysipelas, and diphtheria is not yet complete, though nearly so. Malarial fever also is claimed to belong to the category of known bacterial diseases, but the proofs do not seem as irreproachable to others as they do to their authors.
The entire class of contagious diseases of man can be suspected on just grounds of being of bacterial origin. All analogies, and not a few separate observations, are in favor of this view, while against it no valid argument can be adduced; but it must be admitted that the absolute proof is as yet wanting. Many diseases also, not known to be contagious, like pneumonia, rheumatism, and Bright's disease, have been found associated with parasites, the rôle of which is yet uncertain. It is not sophistry to look forward to an application of the germ-theory to all such diseases, if only for the reason that we know absolutely no other assignable cause, while the changes found in them resemble those known to be due to parasites. In the expectation of all who are not blinded by prejudice, the field is a vast one, which the germ-theory is to cover some day, though progress can only continue if we accept nothing as proved until it is proved.
There can be little doubt that in many, perhaps in most instances, the disease-producing germs enter the body with the air we breathe. At any rate, the organism presents no other gate so accessible to germs as the lungs. Moreover, it has been shown that an air artificially impregnated with living germs can infect animals through the lungs. How far drinking-water can be accused of causing sickness as the vehicle of parasites can not be stated with certainty. There is, as yet, very little evidence to the point, and what there is is ambiguous. Thus, exposed from all quarters to the attacks of these merciless invaders, it seems almost strange that we can resist their attacks to the extent that we do. In fact, one of the arguments used against the germ-theory—a weak one, it is true—is, that, while it explains why some fall victims to the germs, it does not explain why all others do not