72 INFECTIOUS AND PARASITIC DISEASES. 



bacterium. To this class belongs the tetanus bacillus, 

 which, being an obligate anaerobic bacillus (cannot 

 grow in the presence of free oxygen), is compelled to 

 work with another germ that, by absorbing the oxygen, 

 makes the surroundings favorable for its growth. 

 This bacillus also works mischief chiefly when accom- 

 panied by a foreign body, as for example a sliver, powder- 

 grain, etc., or where there has been much laceration 

 of tissues. The same thing is true of the bacillus 

 aerogenes capsulatus. 



There are a few infectious diseases, such 

 Conveyance as malarial fever, yellow fever, and filaria- 

 BY Insects, sis, in which the infectious agents are 



exclusively conveyed by insects. All such 

 diseases are due to animal parasites, which for com- 

 plete development require two hosts, man and an insect. 

 Each host nourishes the parasite during one phase of 

 its growth only, so that in order to reach maturity the 

 latter must pass consecutively from one host to another. 

 Furthermore, since parasites are restricted to especial 

 hosts, the presence of an appropriate host for each one 

 is imperative if they are to be perpetuated. Therefore 

 it follows that the communicability of a disease by an 

 insect is only possible in the event of the presence of 

 that particular variety of insect. So circumscribed is 

 the world of parasitism! For instance, yellow fever 

 is conveyed by only one genus of mosquito (stegomyia 



