582 THE POPULAR SCIENCE MONTHLY. 



trated virulence of the isolated parasite. This bacillus likewise pro- 

 duces spores of a persistent nature, which every consumptive pa- 

 tient spits broadcast into the world. 



Relapsing fever is another disease of definitely proved origin. If 

 we mention, furthermore, abscesses, the dependence of which on bac- 

 teria 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 silk- 

 worm disease and chicken-cholera to the same rank. Several forms of 

 septicemia and pyemia 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 demon- 

 stration 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 role 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 abso- 

 lutely no other assignable cause, while the changes found in them re- 

 semble 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 im- 

 pregnated 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 in- 

 vaders, 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 



