Infectious Agents. 71 



number, provided there be opportunity of convection from their 

 habitat in the soil to the animal body with water, food, etc. They 

 are essentially of telluric origin, and it is customary to speak of 

 spontaneous disease or miasmatic {t6 /jLiaafia, from fxiaivu, to con- 

 taminate) origin in this connection. In the second group are 

 included microorganisms which, being exclusively adapted to the 

 animal body from an indefinite period, live and thrive in it as 

 their habitat (entogeuous infectious agents, obligate parasitic 

 microbes). Diseases occasioned by these do not appear as spon- 

 taneous infections, but occur only when a human being or animal 

 comes in contact with a previously infected individual or with 

 desquamated or excretory material from such an individual in 

 such manner that the infectious agent can pass to the former. 

 Such instances are spoken of as contagious i)i feet ions diseases. 

 [Among many there is to-day a strong tendency to deny the 

 existence of pure miasmatic infections, and thus to regard the 

 terms contagious and infectious as synonymous, such persons hold- 

 ing that the so-called miasmatic diseases are caused by organisms 

 which are essentially facultative parasites, und in more or less 

 direct manner have come from previously diseased individuals. 

 The difiference of view is by no means a vital one, and the 

 author's recognition of their immediate derivation from a source 

 other than a previously diseased individual is correct. In the 

 sense that these facultative parasitic microbes once afifecting an 

 individual may be further transmitted, his division, however, be- 

 comes unnecessary, since in this manner all infectious diseases are 

 contagious.] 



A certain number of infectious maladies may be acquired in 

 both ways ; originating primarily from the soil, the microorgan- 

 isms multiply in the animal body and then are transmitted from 

 the diseased to other individuals (contagio-miasmatic diseases). 



The fact that some microorganisms thrive only in the animal 

 body and die when in the external world is a phenomenon of 

 adaptation. It may be assumed that all these obligate parasitic 

 germs originally lived a free saprophytic life in nature, but having 

 accidentally gained access to an animal body thrived therein and 

 by rapid succession of generations usual to such low organisms 

 acquired a special adaptation for the conditions thus afforded and 

 lost their faculty of propagation outside. 



Changes in nutritional conditions are likely to cause changes 

 in their metabolism, and the influence which invading microbes have 



