130 CONTAGION AND CONTAGIOUS DISEASES. 



existence. For example, septicjemia originates from the acci- 

 dental introduction into the circulation of surrounding external 

 micro-organisms, naturally existing in the air, or within the 

 body itself. Again, tetanus, now recognised as a microbic 

 disease, is induced by an organism located in the ground. These 

 examples might be multiplied, but will be referred to when 

 dealing with microbic diseases specially. It may, however, be 

 mentioned that these so-called non-pathogenic microbes are 

 classified under the term saprogenic or putrefactive bacteria, and 

 acknowledged that they play an important part in the economy 

 of nature. 



Some of these multiply in dead organic matter, others pro- 

 liferate accidentally within the body, their natural habitat being 

 the external surroundings ; but many of them can lead a double 

 existence, saprophytes growing on dead organic matter, and 

 parasites which lead their existence in a living host. 



Parasites which exist in and on living matter only are termed 

 obligatory parasites ; those that lead a double existence facul- 

 tative parasites ; and in addition to these there is another class, 

 called non-contagious facultative parasites, which live in their 

 normal condition in the external media, and only incidentally 

 develop within the living organism, where, not meeting witli 

 favourable conditions for their vitality, their power becomes 

 exhausted, and the disease which they induce is not transmitted 

 irom one animal to another, — such as metritis, omphalitis, 

 arthritis of young animals, tetanus, &c. ; and other diseases 

 described under their various heads are microbic but non- 

 contagious. 



In accordance with the above statement, pathogenic microbes 

 are arranged under these heads — 



I, Contagious obligatory parasites. 

 11. Contagious facultative parasites. 

 III. Non-contagious facultative parasites. 



Of the first it may be stated that their virulence is sometimes 

 diminished, sometimes increased, but never wholly destroyed, 

 whilst passing through the living organism, as in rabies, where 

 the virulence of the yet undiscovered microbe is diminished 

 whilst passing through the body of the monkey and increased 

 in that of the rabbit. Some of them perish immediately on 

 leaving their host, immediate transmission of the virus being 



