170 INFECTION AND IMMUNITY. 



are insusceptible to the ravages of some of the micro-organisms which 

 readily incite infectious disease in man. Thus the lower animals are, 

 so far as we know, naturally immune to syphilis. To certain diseases of 

 the lower animals, on the other hand, man is not subject. But to certain 

 other infectious diseases, tuberculosis for example, both men and lower 

 animals are susceptible, and both are, in fact, under the prevailing con- 

 ditions of modern life, frequent victims. 



So far as the liability to the transmission of the infectious agents from 

 man to man is concerned, there is a very marked and significant differ- 

 ence between the infectious diseases. It is common usage to speak of the 

 transmission or communication of disease, as if disease were a self -exist- 

 ent thing. This usage fosters much loose thinking. What we call dis- 

 ease is a process involving a departure from, failure in, or perversion 

 of normal physiological action, either in the material constitution or in 

 the functional integrity of the living organism. When, therefore, we 

 speak of the transmission or communication of disease, what we really 

 mean is not that the disease, but the agent capable under suitable condi- 

 tions of inciting the disease is transmitted or communicated. If we hold 

 this obvious implication in mind, it is useful to group the infectious dis- 

 eases of man into two great primary classes: 1st, Those which under the 

 usual conditions of life are not readily communicable. 2d, Those which 

 under the usual conditions of life are readily communicable. 



But while, for convenience, we may speak of non-communicable and 

 communicable diseases, we should remember that these two classes merge 

 into one another, and that in fact the agents of infection may at least 

 artificially in all cases be conveyed from one individual to another. It 

 is only when the conveyance under natural conditions occurs in a round- 

 about way, or through intermediary agencies, such as the mosquito for 

 example, in malaria or yellow fever, that one may advisedly speak of 

 non-communicable infectious diseases. 



Among the communicable infectious diseases there exists the widest 

 difference in the liability to transmission under ordinary circumstances. 

 Thus the infectious agents in smallpox and scarlatina are given off from 

 the body under such conditions as render possible and frequent their 

 direct transmission through the air to another individual. In syphilis, 

 tetanus, and rabies, on the other hand, transmission of the infectious 

 material is rare or impossible without a direct inoculation. 



Between these extremes the widest diversity exists in the liability to 

 transmission of the infectious agents of the diseases of this class. In 

 fact the liability to infection on the part of a healthy individual in the 

 presence of a victim of infectious disease is largely dependent upon the 

 intelligent care which is exercised in the disposition of the material con- 

 taining the pathogenic micro-organism which in one way or another the 

 infected body sets free. 



So that while it may be useful to arrange the communicable infectious 

 diseases in groups, or in such serial order as may indicate the degree of 

 commuuicability of each under the ordinary conditions of life, it should 



