INFECTION. 33 



substance present in the expressed cell-fluid, which pos- 

 sesses the properties of fermenting sugar, as parablastic, 

 and designates it zymase. This zymase is produced within 

 the cell, and H. Buchner compares the entire process with 

 the production and the action of the toxins of tetanus and 

 diphtheria, which likewise are formed within the bacterial 

 cell. 



Some species of bacteria are capable of thriving not only 

 upon dead, but also upon living, matter, in man as well as 

 in animals. Gaining entrance into a living organism, these 

 bacteria multiply at the expense of their host, and, with 

 the aid of their metabolic products, unfold their deleterious 

 activity ; the affected individual is made ill. Bacteria capa- 

 ble of inducing disease are designated pathogenic ; those, 

 on the other hand, that are innocuous and harmless are 

 designated nonpathogenic. Those that are capable of under- 

 going multiplication only within a higher living organism 

 are known as parasites (true, strict, obligate parasites). In 

 contrast with these are saprophytes, those bacteria that 

 thrive only upon dead material. There is, however, no 

 sharp division between parasites and saprophytes. Many 

 bacteria are adapted to both modes of life : these are facul- 

 tative parasites that is, they live only temporarily within 

 the animal body, but usually external to it, in the earth or 

 in water. On the other hand, many of the especially patho- 

 genic bacteria are essentially parasitic. By means of our 

 culture-media we have, however, succeeded in cultivating 

 them outside the body, and have thus, by artificial means, 

 made them facultative saprophytes. 



IL INFECTION. 



Those diseases are designated infectious that result 

 through the vital activity of vegetable or animal micro- 

 organisms. The causative agents of the majority of the 

 diseases included in this group belong to the class of bac- 

 teria. The filamentous fungi, as well as the lower forms 

 of animal life (protozoa), have until now played a less 

 important role in the etiology of disease. Formerly, the 

 infectious diseases were subdivided into contagious and 

 miasmatic. With the former, transmission takes place 

 through' contagion that is, through the conveyance of the 

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