i6 



Insects and Disease 



are caused by animal parasites is contagious, while 

 many of those caused by bacteria are both con- 

 tagious and infectious. 



Just over on the plant side of this indefinite 

 border line are the minute organisms known as 

 bacteria. Their numbers are infinite and they are 

 found everywhere. The majority of them are 

 beneficial to mankind in one way or another, but 

 some of them cause certain of the diseases that we 

 will have to discuss later so attention may be called 

 here to a few of the important facts in regard to 

 their organization and life-history in order that we 

 may better understand how they may be so easily 

 transferred from one host to another. 



Although these bacilli are so extremely minute 

 (Fig. 7), some of them so small that they cannot be 

 seen with the most powerful microscopes, they 

 differ in size, shape, methods of division and spore- 

 formation. Each species makes a characteristic 

 growth on gelatin, agar or other media upon which 

 it may be cultivated. In this way as well as by the 

 inoculation of animals the presence of the ultra- 

 microscopic kinds may be demonstrated. 



The method of reproduction is very simple. 

 They increase to a certain point in size, then divide. 

 This growth and division takes place very rapidly. 

 Twenty to thirty minutes is sufficient time in some 



