(Jontagious a/iid Epizootic Diseases. 47 



and thus inducing a disease whicli, in the great majority of 

 cases, was slight and comparatively harmless. In the same 

 way children have been voluntarily exposed to the infection 

 of measles or scarlet fever when that particular disease was 

 prevailing in an unusually mild form, and by passing through 

 such mild form of the malady have been empowered to resist 

 the infection when at a later date the disease had assumed 

 a malignant and fatal type. 



Of late years facts have accumulated which tend to 

 throw light on the real cause of such acquired immunity. 

 To comprehend these it is necessary to state one or two 

 fundamental truths. 



1. A contagious disease is maintained and propagated in 

 an animal body, and from one animal to another, by the 

 multiplication and transference of a living organism, having 

 the property possessed by all living bodies of increase by 

 natural generation, of assimilation of food, and of the ex- 

 cretion of waste material. In a certain number of conta- 

 gious diseases these have been shown to be infinitesimal 

 cellular organisms (bacteria) allied to the ferments which 

 produce alcohol, vinegar, the carbonic acid which raises 

 bread, and the offensive liquids and gases of putrefaction. 

 It is not necessary to claim that all contagious diseases are 

 caused by bacterial ferments ; it is enough for our present 

 purpose to assume that every contagious disease is due to 

 the presence of a distinct microscopic living particle which 

 feeds, excretes, and increases by generation as do ferments. 

 The only other alternative, that it is due to a chemical 

 agent which acts injuriously on the tissues of the body, dis- 

 proves itself ; for every chemical agent expends its power in 

 exercising such chemical action, and can by no means re- 

 cruit its substance nor strength, but will act with greater or 

 less effect according to the amount originally applied, and 

 must be more speedily exhausted in exact ratio with the bulk 

 or the number of the animals attacked ; whereas the disease- 



