98 BACTERIOLOGY. 



gle streptococcus may, through its rapid multiplication, 

 produce death in eighteen hours ; a single tubercle 

 bacillus, on the other hand, cannot produce sufficient 

 numbers in less than two weeks. The virulence of the 

 septicsemic class of bacteria is not at all the same when 

 measured in different animals, and it is largely for this 

 reason that the virulence in test animals does not 

 usually correspond with the severity of the case from 

 which the organism was derived. We should re- 

 member in this connection the varying power of resist- 

 ance in different animals and of the same individual 

 at different times. 



Mixed Infection. The combined effect upon the tissues 

 of the products of two or more varieties of pathogenic 

 bacteria, and also of the influence of these different forms 

 on each other, are of great importance in the produc- 

 tion of disease. The infection from several different 

 organisms may occur at the same time, or one may fol- 

 low the other or others so-called secondary infection. 

 Mixed infection arises usually from the inoculation of 

 more than one variety of bacteria simultaneously. 

 Thus, an abscess is often due to several forms of 

 pyogenic cocci. If a wound is infected from such a 

 source the inflammation produced will probably be 

 caused by all the varieties present in the original in- 

 fection. Peritonitis following intestinal injuries must 

 necessarily be due to more than one organism. Thus, 

 whenever two or more varieties of bacteria are trans- 

 ferred to a new soil, mixed infection takes place if 

 more than one variety is capable of developing in that 

 locality. 



Forms of infection which are allied to both mixed 

 and secondary infection are those occurring in the 



