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 
septiceemic 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 
